Under what circumstances can the new Brexit extension (31 Oct) be ended short of that date?

The EU has [said](https://www.consilium.europa.eu/en/meetings/european-council/2019/04/10/) that:

> If the withdrawal agreement is ratified by both parties before this date, the withdrawal will take place on the first day of the following month.

But the BBC also [reports](https://www.bbc.co.uk/news/uk-politics-47889404) that:

> The UK must now hold European elections in May, **or leave on 1 June** without a deal.

Is a no-deal scenario before the 31st of October really a possibility?  
Are there other circumstances – apart from ratification of the withdrawal deal in Parliament, and not organising EU Parliament elections – under which the UK might leave the EU before that date?

By what date would it become certain that the UK is, or isn't, taking part in European elections?