Theresa May is to hold last-minute Brexit talks with the leaders of Germany and France later, four days before the UK is due to leave the EU.
Mrs May is meeting Angela Merkel in Berlin, followed by Emmanuel Macron in Paris, to urge them to back her request to delay Brexit again until 30 June.
The prime minister will be at an emergency summit on Wednesday when all EU states will vote on an extension.
Cross-party talks aimed at breaking the impasse are also set to continue.
The negotiating teams will be joined by Chancellor Philip Hammond and shadow chancellor John McDonnell, with the Labour frontbencher saying they hoped to “broaden the talks”.
The UK is currently due to leave the EU at 23:00 BST on Friday.
So far, MPs have rejected the withdrawal agreement Theresa May reached with other European leaders last year.
One of most contentious parts of the plan is the Irish backstop – an insurance policy that aims to prevent a hard border returning to the island of Ireland.
The EU has continually said it will not re-open the withdrawal agreement for negotiations, but Leader of the Commons Andrea Leadsom renewed her plea for them to look again.
She told reporters the “best possible outcome” would be if the “EU decide to support measures on the backstop”, and that she though it would be “fantastic [if] Angela Merkel will try to support a proper UK Brexit by agreeing to open the withdrawal agreement”.
On Monday evening, Parliament passed a bill brought by Labour MP Yvette Cooper, which aims to force the prime minister to request a Brexit extension – rather than leave the EU without a deal on Friday, which is the default position.
The government opposed the bill, saying it was unnecessary as Mrs May was already seeking an extension. But the backbenchers behind it wanted to ensure it became law to prevent any changes in her strategy.
The bill received its Royal Assent on Monday night, and Commons Leader Andrea Leadsom told MPs that this meant there will be a government motion on Tuesday asking the House to approve the PM’s request to the EU to delay Brexit.
The final decision on an extension lies with the EU – and the leaders of all the 27 other EU countries have to decide whether to grant or reject an extension.
If the UK is still a member of the EU on 23 May, it will have to take part in European Parliamentary elections.
Germany’s Europe Minister Michael Roth said Mrs May would receive a warm welcome in Berlin, but his government’s priority was maintaining the unity of the European Union.
On Monday, Mrs May spoke by phone to the Dutch Prime Minister Mark Rutte, who said it was “crucial” for the EU’s members to know “when and on what basis” the UK will ratify the withdrawal deal.
Luxembourg’s Foreign Minister Jean Asselborn said the UK would “certainly not” leave without a deal on Friday.
But Ireland’s Deputy Prime Minister Simon Coveney said a no-deal Brexit was still possible – even though it would represent “an extraordinary failure of politics”.
EU leaders are curious to hear the prime minister’s Plan B. They hope there is one, although they’re not convinced.
They want to know, if they say yes to another Brexit extension, what it will be used for.
And they suspect Theresa May wants them to do her dirty work for her.
EU diplomatic sources I have spoken to suggest the prime minister may have officially asked the EU for a short new extension (until 30 June) as that was politically easier for her back home, whereas she believed and hoped (the theory goes) that EU leaders will insist instead on a flexible long extension that she actually needs.
The bottom line is: EU leaders are extremely unlikely to refuse to further extend the Brexit process.
Talks between the government and Labour will continue this afternoon.
Justice Secretary David Gauke told BBC Radio 4’s Today programme the discussions had been “undertaken in a constructive manner from both sides”.
He said: “There are some areas of common ground, but there are also well-known areas of disagreement.
“But there is a lot of work that’s going on at the moment in terms of identifying perhaps where we can forward.”
Mr Corbyn said that the ongoing talks “have to mean a movement” in the government’s policy, adding: “So far there’s been no change in those red lines.”
On Monday, sources indicated the PM had not accepted Labour’s customs union demand, but there was a move towards changing the non-binding political declaration – the part of Mrs May’s deal outlining the future relationship with the EU.
The government also reportedly suggested offering Labour a guarantee that any deal they reached could not be undone by the next Conservative leader, creating a “lock”.
But BBC political editor Laura Kuenssberg said there was “deep concern” on the Labour side that any legal promise could be undone in the future.
The prime minister has been warned by members of the 1922 committee of Conservative backbenchers that agreeing a customs union with the EU in Brexit talks would be “unacceptable”.