The BBC has an analysis article on the Chagos Islands dispute, from which I'll quote a few excerpts in an attempt to address your questions.
How true is this claim?
It's disputed. The UK says it's it's in the right, quoting from the article:
Britain's Foreign and Commonwealth Office (FCO) insists it has every right to hold onto the islands - one of which, Diego Garcia, is home to a US military airbase.
"The UK has no doubt as to our sovereignty over the British Indian Ocean Territory (BIOT), which has been under continuous British sovereignty since 1814," it said in a statement.
"Mauritius has never held sovereignty over the BIOT and the UK does not recognise its claim."
This claim is disputed by some in the UK, for example opposition leader Jeremy Corbyn, again quoting from the article:
But Labour Party leader Jeremy Corbyn said it was important to return the islands "as a symbol of the way in which we wish to behave in international law".
He added: "I am looking forward to being in government to right one of the wrongs of history."
Mauritius disputes this as well, and many members of the UN General assembly sided with Mauritius. Again, quoting from the Article:
The Chagos Archipelago was separated from Mauritius in 1965, when Mauritius was still a British colony. Britain purchased it for £3m - creating the BIOT.
Mauritius claims it was forced to give it up in exchange for independence, which it gained in 1968.
In May, the UN General Assembly voted overwhelmingly in favour of the Chagos Islands being returned - with 116 states backing the move and only six against.
The UN said that the decolonisation of Mauritius "was not conducted in a manner consistent with the right to self-determination" and that therefore the "continued administration... constitutes a wrongful act".
This follows after an International Court of Justice ruling earlier in 2019. From a previous article by the BBC reporting on that:
The International Court of Justice said the islands were not lawfully separated from the former colony of Mauritius.
And further in the article, the ruling is elaborated:
Judge Abdulqawi Ahmed Yusuf described the UK's administration of the Chagos Islands - located more than 2,000 miles off the east coast of Africa - as "an unlawful act of continuing character".
He added the UK was "under an obligation to bring an end to its administration of the Chagos Archipelago as rapidly as possible".
The advisory opinion by the International Court of Justice is very long. It considers a lot of evidence but it seems to be based, at least in my reading as a non-lawyer, around the principle of self-determination. The term
self-determination appears 57 times in the advisory opinion. I will cite the last mention of self-determination in the ruling:
- Since respect for the right to self-determination is an obligation erga omnes, all States
have a legal interest in protecting that right (see East Timor (Portugal v. Australia), Judgment,
I.C.J. Reports 1995, p. 102, para. 29; see also Barcelona Traction, Light and Power Company,
Limited (Belgium v. Spain), Second Phase, Judgment, I.C.J. Reports 1970, p. 32, para. 33). The
Court considers that, while it is for the General Assembly to pronounce on the modalities required
to ensure the completion of the decolonization of Mauritius, all Member States must co-operate
with the United Nations to put those modalities into effect. As recalled in the Declaration on the
Principles of International Law concerning Friendly Relations and Co-operation among States in
accordance with the Charter of the United Nations
“Every State has the duty to promote, through joint and separate action,
realization of the principle of equal rights and self-determination of peoples, in
accordance with the provisions of the Charter, and to render assistance to the
United Nations in carrying out the responsibilities entrusted to it by the Charter
regarding the implementation of the principle” (General Assembly
resolution 2625 (XXV)).
What is the history here?
The history is explained thoroughly in the advisory opinion by the International Court of Justice (I only quote points I think are relevant; the list of historic facts goes on for quite a while, I suggest reading it in the PDF if you're interested):
Between 1814 and 1965, the Chagos Archipelago was administered by the
United Kingdom as a dependency of the colony of Mauritius. From as early as 1826, the islands of
the Chagos Archipelago were listed by Governor Lowry-Cole as dependencies of Mauritius. The
islands were also described in several ordinances, including those made by Governors of Mauritius
in 1852 and 1872, as dependencies of Mauritius. The Mauritius Constitution Order of 26 February
1964 (hereinafter the “1964 Mauritius Constitution Order”), promulgated by the United Kingdom
Government, defined the colony of Mauritius in section 90 (1) as “the island of Mauritius and the
Dependencies of Mauritius”.
In accordance with General Assembly resolution 66 (I) of 14 December 1946, the
United Kingdom as the administering Power regularly transmitted information to the General
Assembly under Article 73 (e) of the Charter of the United Nations concerning Mauritius as a nonself-governing territory. The information submitted by the United Kingdom was included in several
reports of the Fourth Committee (Special Political and Decolonization Committee) of the General
Assembly. In many of these reports, the islands of the Chagos Archipelago, and sometimes the
Chagos Archipelago itself, are referred to as dependencies of Mauritius. In its 1947 Report,
Mauritius is described as comprising the island of Mauritius and its dependencies among which are
mentioned the island of Rodriguez and the Oil Islands group of which the principal island is Diego
Garcia. The Report of 1948 collectively referred to all of the islands as “Mauritius”. The Report
of 1949 states that “there are dependent upon Mauritius a number of islands scattered over the
Indian Ocean, of which the most important is Rodriguez . . . Other dependencies are: Chagos
Archipelago . . . Agalega and Cargados Charajos”.
In February 1964, discussions commenced between the United States of America
(hereinafter the “United States”) and the United Kingdom regarding the use by the United States of
certain British-owned islands in the Indian Ocean. The United States expressed an interest in
establishing military facilities on the island of Diego Garcia.
On 29 June 1964, the United Kingdom also commenced talks with the Premier of the
colony of Mauritius regarding the detachment of the Chagos Archipelago from Mauritius. At
Lancaster House, talks between representatives of the colony of Mauritius and the United Kingdom
Government led to the conclusion on 23 September 1965 of an agreement (hereinafter the
“Lancaster House agreement”, described in more detail in paragraph 108 below).
On 8 November 1965, by the British Indian Ocean Territory Order 1965, the
United Kingdom established a new colony known as the British Indian Ocean Territory (hereinafter
the “BIOT”) consisting of the Chagos Archipelago, detached from Mauritius, and the Aldabra,
Farquhar and Desroches islands, detached from Seychelles.
On 16 December 1965, the General Assembly adopted resolution 2066 (XX) on the
“Question of Mauritius”, in which it expressed deep concern about the detachment of certain
islands from the territory of Mauritius for the purpose of establishing a military base and invited the
“administering Power to take no action which would dismember the Territory of Mauritius and
violate its territorial integrity”.
On 20 December 1966, the General Assembly adopted resolution 2232 (XXI) on a
number of territories including Mauritius. The resolution reiterated that
“any attempt aimed at the partial or total disruption of the national unity and the
territorial integrity of colonial Territories and the establishment of military bases and
installations in these Territories is incompatible with the purposes and principles of the
Charter of the United Nations and of General Assembly resolution 1514 (XV)”.
- Based on the 1966 Agreement, the United States and the United Kingdom agreed that the
Government of the United Kingdom would take any “administrative measures” necessary to ensure
that their defence needs were met. The Agreed Minute provided that, among the administrative
measures to be taken, was “resettling any inhabitants” of the islands. The inhabitants of the Chagos
Archipelago are referred to as Chagossians and, sometimes, as the “Ilois” or “islanders”. In this
Opinion these terms are used interchangeably.
- On 7 August 1967, general elections were held in Mauritius and the political parties in
favour of independence prevailed.
- Between 1967 and 1973, the entire population of the Chagos Archipelago was either
prevented from returning or forcibly removed and prevented from returning by the
United Kingdom. The main forcible removal of Diego Garcia’s population took place in July and
And, according to Wikipedia:
Diego Garcia is an island of the British Indian Ocean Territory, an overseas territory of the United Kingdom. It is a militarised atoll just south of the equator in the central Indian Ocean, and the largest of 60 small islands comprising the Chagos Archipelago.
In March 1971, United States Naval construction battalions, (Seabees), arrived on Diego Garcia to begin the construction of the communications station and an airfield.
What are the consequences of ignoring a UN resolution?
That depends. As noted in Sjoerd's answer, the UN General Assembly cannot enforce it resolutions and the UK has a veto in the Security Council which can enforce its resolutions.
That said, the reason it comes up now, according to Southern Africa correspondent Andrew Harding for the BBC:
Over the decades Mauritius has staked its claim, and finally - particularly after the Brexit vote - Britain's traditional allies in the international community have started to desert Britain, to abstain or to vote against it at the UN.
And the UN is now taking pretty significant steps to say: "Britain you are behaving appallingly, this is still colonialism - give it back."
Britain has ignored those calls - so what might any repercussions look like?
Sanctions would be slow, incremental and largely institutional - in the sense that Britain is going to find itself squeezed at institutions that it has traditionally seen as very important.
Britain no longer has a judge on 14-seat International Court of Justice in The Hague, and it's going to start to see UN maps reflecting the legal fact that the UN sees this islands as belonging to Mauritius.
The deadline is not binding, so no sanctions or immediate punishment will follow - but that could change.
So while it's not directly enforceable, it may be seen as part of a geopolitical shift. Some countries that have historically sided with the United Kingdom may look for other alliances in which there is more to gain for them.