Brexit vote: What do the changes to Theresa May's deal mean and what happens if MPs reject it?

European Commission president Jean-Claude Juncker insists 'there will be no new negotiations' – so what happens next?

MPs will vote tonight on an “improved Brexit deal” after last-minute negotiations in Strasbourg, Theresa May has insisted.

Tuesday evening’s Commons showdown – the second “meaningful vote” after the prime minister’s first attempt was defeated by a margin of 230 – comes just 17 days before the UK is scheduled to leave the EU.

The prime minister said she "passionately believed" her Brexit deal addressed concerns raised by MPs who feared the backstop – the legal guarantee to prevent a hard border on the island of Ireland no matter the outcome of negotiations – would keep the UK in a customs arrangement with the EU indefinitely.

But how is the latest iteration of her agreement different to the one rejected by MPs in January, and what happens next?

What are the changes?

Ms May has agreed three new documents with European Commission president Jean-Claude Juncker.

The first is a "legally binding joint instrument" relating to the Withdrawal Agreement – the nearly 600 page document that, if it is approved by parliament, will set out the terms on which the UK will leave the EU.

The government states the new document "reduces the risk that the UK could be deliberately held in the Northern Ireland backstop indefinitely and commits the UK and EU to work to replace the backstop with alternative arrangements by December 2020".

The second new document is branded a "unilateral declaration by the UK" which sets out "the sovereign action the UK would take to provide assurance that the backstop would only be applied temporarily".

The final text is a supplement to the Political Declaration – the document setting out in broad terms how the UK's relationship with the bloc after Brexit will look – "setting out commitments by the UK and the EU to expedite the negotiation and bringing into force of their future relationship".

What did Ms May promise after her last Commons defeat?

On 29 January, MPs, including the prime minister, voted for an amendment put by the Tory backbencher Sir Graham Brady that instructed the government to replace the backstop with “alternative arrangements”.

Immediately after that vote, the EU shot down any hopes of that being achieved. “The backstop is part of the withdrawal agreement, and the withdrawal agreement is not open for re-negotiation,” a spokesperson said.

On Monday night, the de facto deputy prime minister, David Lidington, said the new documents gave "confirmation that the EU cannot try to trap the UK in the backstop indefinitely and that doing so would be an explicit breach of the legally-binding commitments that both sides have agreed".

But critics of Ms May’s revised deal were quick to claim she had failed to deliver a replacement to the backstop, as promised.

Jeremy Corbyn, the Labour leader, said: “This evening’s agreement with the European Commission does not contain anything approaching the changes Theresa May promised parliament, and whipped her MPs to vote for."

Keir Starmer, the shadow Brexit secretary, said he would be “surprised” if the changes were enough to allow the attorney general, Geoffrey Cox, to change his legal advice relating to the backstop – a key measure Brexit-backing MPs have said will inform how they will vote on Tuesday evening.

And hardline Tory Eurosceptic Steve Baker said: "Even by the government's own standards I think this falls very far short of what the government whipped us to vote for."

Others have been more equivocal.

David Davis, the former Brexit secretary, said the revised deal was "just about acceptable to me" but depended "very, very heavily on a robust, clear response" from the attorney general.

What comes next?

If MPs back Ms May’s deal tonight, the UK will leave the EU on or close to 29 March 2019, dependant on the speed with which the necessary legislation can be passed.

If MPs vote the deal down, the scale of the defeat is likely to influence what course Ms May takes.

It is too early to say what this would be, aside from a promise to consult MPs on whether they support leaving the EU at the end of this month without a withdrawal agreement or framework for a future relationship in place.

That vote on a no-deal Brexit will take place on Wednesday, the prime minister has said.

The following day, MPs will be given a vote on whether to ask the EU for a “short, limited extension to article 50”, delaying Brexit.

But neither of these two subsequent votes will offer any solutions to the fundamental question of how – or whether – the UK should leave the EU.

There had been a number of machinations talked about as potential options for Ms May under these circumstances, including a possible third meaningful vote.

However Mr Juncker was clear on Monday evening that this revised settlement would be the last and that the EU would not agree to any further changes.

“There will be no new negotiations. It is this,” he said.

"In politics, sometimes you get a second chance. It is what we do with the second chance that counts. Because there will be no third chance.

"There will be no further interpretation of the interpretations and no further assurances on the reassurances.

"Let us be crystal clear about the choice - it is this deal or Brexit might not happen at all."