The Letwin amendment has been passed by parliament by a margin of 322 to 306, delaying the vote on British prime minister Boris Johnson's Brexit deal.  But what is it?

Who is Oliver Letwin?

A former adviser to Margaret Thatcher and a Conservative MP since 1997, Sir Oliver Letwin is among many MPs to have been ousted by Johnson when they had the whip removed – effectively expelling them from the party, although they keep their seat in parliament – over opposition to Johnson's plans. 

Letwin, who has announced he will stand down at the end of this parliament has been a chief architect of parliamentary legislation aimed to prevent, or at least slow down, the no-deal Brexit favored by hardline Brexiteers. 

How do amendments work?

Filed before parliament votes on a government-suggested bill (which is what draft legislation is called before it is passed into law), amendments are proposals to change, remove or augment their wording. In this way, they act as a parliamentary check on government's intended legislation. 

Any number of amendments can be put forward, but only a certain number are voted on. This selection process is done by the Speaker of the House, currently John Bercow, who reveals at the start of the bill's reading which amendments the house will vote on. 

What is the Letwin amendment?

Parliamentary amendments can be shrouded in unclear language but in essence, Letwin's amendment withholds parliamentary approval for Johnson's deal until the legislation implementing the UK's withdrawal has become law. 

Letwin has called the amendment an "insurance policy" to create an extension. He argues it is necessary to prevent the UK unintentionally crashing out without a deal. Some MPs suspect that if Johnson's deal is passed today, the government would then be free of the current restrictions on it being legally able to pull Britain out of Europe without a deal. 

What now Letwin's amendment has passed?

The government is now bound by the provisions of the earlier Benn Act – already voted into the country's legislation, as noted by its title as an Act (of law). Officially known as the European Union (Withdrawal) (No 2) Act, the Benn Act says should prevent Johnson from presenting MPs with a binary choice of his deal and a no-deal Brexit.

Instead, the government is legally bound to ask the EU for an extension – before 11pm on Saturday evening. However, Johnson has insisted he will not do that: "I will not negotiate a delay with the EU and neither does the law compel me to do so."

What if Letwin's amendment hadn't passed?

A no-deal Brexit would have been a possibility, but not a certainty: the government could still have sought parliamentary approval for Johnson's deal. However, as Letwin's amendment is essentially a braking mechanism, rejecting it would presumably have sped Brexit towards a conclusion, one way or the other.

What happens next?

Ever since the initial referendum, which was expected to be a relatively easy win for the Remain camp and arguably only ever designed to end the debate on European membership, it has been a fool's errand to make predictions around Brexit. 

The decision on Johnson's deal is expected to be so incredibly tight that it may even go down to two or three votes either way... if it even takes place today. Sources at 10 Downing Street said earlier that if the Letwin amendment passes, MPs would be sent home: "A vote for Letwin means MPs voting to render the entire day, that they demanded, meaningless." Which is what happened.

It is expected they will likely vote on Johnson's deal on Tuesday instead. What that means for the Benn Act – and Johnson having to request a deadline extension – remains to be seen on a fascinating day in parliament.