What is the Letwin amendment threatening to derail Boris Johnson's Brexit deal?

MPs will today vote on two amendments in connection with Boris Johnson's bid to get his Brexit deal with the EU approved.

Speaker John Bercow announced the two amendments for consideration at the start of the Commons today.

The first is the amendment from Independent Sir Oliver Letwin, the former Cabinet minister who had the Tory whip withdrawn after rebelling over Brexit, which proposes to withhold approval for the deal unless and until legislation implementing it is passed.

The second for MPs to consider will be a cross-party amendment rejecting a no-deal Brexit and seeking a second referendum.

Letwin's proposal states that the House "withholds approval" of the deal until such time that the legislation to enact the deal has been passed.

The reason for this is that a straightforward approval would prevent the Benn Act - which obliges the PM to request an Article 50 extension if he does not have an approved deal - from kicking in.

The Letwin amendment means Mr Johnson would be forced to apply for an extension regardless of the result.

This is voted on first and has the backing of Labour and the Tory rebels, so could pass.

Unless Mr Johnson finds a way to ignore it, he may be in the embarrassing position of having to ask for a Brexit delay beyond October 31.

Mr Letwin said it was an "insurance policy" to prevent Britain "crashing out" without a deal on Oxford.

But Government sources reportedly warned that if it passed, Tory MPs will essentially abstain on the final vote and the Withdrawal Agreement Bill will be introduced next week.