Is there any legal ground to prosecute a politician for lying to the public where their lie gave them political gain?
Yes, see Timothy Morrison and others v Alistair Carmichael MP and Alistair Buchan in 2015.
Alastair Carmichael escaped punishment when it had not been proved beyond reasonable doubt that he had committed an "illegal practice" at an Election Court in 2015 after a case was brought against him by several of his constituents over remarks made prior to the 2015 general election in the wake of a false leak from the Scotland Office which he headed alleging comments made by First Minister Nicola Sturgeon to the French ambassador.
While the judges found that he lied (in fact told a "blatent lie"), and did so for political gain, he got off on the technicality that the lies were not considered to be "in relation to [his] personal character or conduct", which is part of the test for the offence under the Representation of the People Act 1983.
“It is of the essence of section 106 that it does not apply to lies in general: it applies only to lies in relation to the personal character or conduct of a candidate made before or during an election for the purpose of affecting that candidate’s return,” Lady Paton said.
We're going to find out, in Ball v Johnson. This case is a private prosecution of Boris Johnson for alleged misconduct in public office (as MP and Mayor of London - two public offices). Misconduct in public office is a criminal offence with a maximum punishment of life imprisonment. The alleged misconduct includes lying in statements of fact on a number of occasions about the UK's monetary contributions to the EU budget.
I imagine the court will criticise Johnson but say it is for the normal democratic process, not the courts, to regulate such political speech, even misleading or false speech. That he can be voted out by his constituents at the next election or (as MP) held to account by the Committee for Standards in Public Life. However, that is just my speculation.
Here are the decision and reasons for granting the summons in the case.
However the High Court disagreed with the district court judge. Apparently, the judge agreed with Johnson's lawyer that the prosecution was "vexatious" and "politically motivated". The High Court dismissed the case and quashed the summons.
Under very specific circumstances, there is already case law that some lies for gain during elections can be illegal.
In the 2010 General Election Phil Woolas originally won the seat of Oldham East and Saddleworth in an extremely ill-tempered contest against the Liberal Democrat Elwyn Watkins. Subsequently Watkins issued a petition against the result under section 106 of the Representation of the People Act about election leaflets implying he was a friend to terrorists and taking slush money from foreign powers. An election court found in Watkins' favour, and a new by-election was called, with Woolas now disqualified from holding elected office.
Political speeches are not given under oath, and there is no general ban on lying, using deceptive speech, bending the truth, being economical with the truth. These are not explicitly disallowed.
The Johnson case makes an interesting argument, that lying is a form of "misconduct in public office". This is, as yet, untested. Existing precedent is that political statements are not treated specially. There are particular laws against libel and slander, but these apply to everyone, not just politicians. It is up to the voters to decide if they believe a politician and vote accordingly.
There is a principle of "Freedom of expression" which is in article 10 of the Human rights act. It is a right to hold an opinion and to express it, even if that opinion is "wrong". It can be very hard to distinguish between a "lie" and "being wrong". Even harder to distinguish between a "lie", and a deliberate oversimplification to make a rhetorical point.
Perhaps until it is tested in court in Ball v Johnson we will not know whether it is legal or not.
However, the way I see it is that hyperbole and platitudinous argument are one thing, but when you go so far as to paint an erroneous fact, and one that is markedly relevant to a political campaign, on the side of a bus as though it were uncontested truth, I do believe you have crossed a line. And if it is done, knowingly, by a public official in that capacity, it surely amounts to misconduct, doesn't it?
This could become a very important case in confirming where the law stands on matters of fake news, as well as where political campaigning ends and misconduct begins. For if it is found NOT to be misconduct, given the ease of publication that exists today, it would seem to me to give a legal blessing to heavily funded fake news. So that in the end what amounted to "the truth" would be what someone had obtained sufficient funding to disseminate - an Orwellian prospect if ever there was one.
Marcus Ball is taking Boris Johnson to court, for misconduct in public office, and the alleged misconduct happened by Johnson lying about the amount of the money the UK sends to the EU.
He is not taken to court just for lying, so it looks like Marcus Ball doesn't believe that "politicians lying to the public for political gain" is something you can win in a court case, at least not in the case of Boris Johnson. So as far as the question is concerned, the answer would be "usually no". There have to be added elements. The lying could be fraud, or as claimed here, "misconduct in public office", but usually the lying alone is not enough.
On the other hand, the court has accepted the case. I think the only claimed wrongdoing is the lying, so it seems that the court assumes lying to the public can in the right circumstances be a "misconduct in public office" and punishable. Again to the question: Yes, lying to the public for political gain can in the right circumstances be illegal "misconduct in public office".
What's open is whether Marcus Ball can prove that Johnson was lying (that would be the easy part), and that he can convince the court that this particular lying would be "misconduct in public office". Johnson's lawyers seem to claim that a requirement for "misconduct in public office" is abuse of power of the office (or failure to use the power of the office which wouldn't be the case here). If that theory is right, then Ball would have to prove that the lying was "abuse of the power of the office".
PS. Case thrown out. Johnson keeps insisting that black is white.