In a highly unusual move, Unite boss Len McCluskey, a close ally of Jeremy Corbyn, visited No 10 for high-level discussions, as well as TUC general secretary Frances O'Grady, Dave Prentis, of Unison, and GMB chief Tim Roache.
The prime minister is scrambling to win political backing for a Brexit plan that could secure a Commons majority after her deal was overwhelmingly rejected by MPs.
And on Thursday, cabinet minister Amber Rudd repeatedly refused to say whether she would resign from her government role to stop a no-deal Brexit, just hours after Airbus issued a stark warning over the impact of a disorderly Brexit on its business in the UK.
During an interview on BBC Two’s Newsnight, Ms Rudd, the work and pensions secretary said she was going to “wait and see” if the prime minister allows a free vote on a series of amendments in the Commons next week.
Asked three times whether she would quit the Cabinet to back a bid by Labour's Yvette Cooper to extend Article 50 unless a deal is reached by the end of February, and she refused to rule it out.
She said: "At this stage I'm going to stick to trying to persuade the Government to allow it to be a free vote. There is a lot taking place and there are a lot of new amendments. We'll have to wait and see."
Ms Rudd also told the programme she is "committed to making sure we avoid no-deal", which she described as "the worst possible outcome”.
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Welcome to The Independent's politics liveblog, where we will be bringing you the latest developments throughout the day.
Theresa May is due to meet trade union bosses today, as part of efforts to break the Brexit deadlock. Len McCluskey, the powerful Unite boss and ally of Jeremy Corbyn, will be among the left-wing union leaders meeting the PM.
A Downing Street spokesman said: "The PM has said that this is a process of engagement across the House of Commons but also with other interested sectors.
"She has spoken to business leaders and she will be talking with union leaders.
"Issues I expect to be discussed will be around employment rights, environmental standards and those sorts of things."
My colleague Ben Kentish has written a timely explainer on the Brexit views of the major trade unions.
Annegret Kramp-Karrenbauer, who's succeeded Angela Merkel as leader of Germany's CDU, has said it is "not clear what Great Britain wants" on Brexit.
Here's the clip of her speaking to Bloomberg in Davos:
Breaking story: Scotland's former first minister Alex Salmond has been arrested and charged, police have said.
The head of plane giant Airbus has branded Theresa May’s handling of Brexit a “disgrace” and ramped up a threat to close its plants if the UK crashes out of the EU with no deal.
Tom Enders, the company’s chief executive, urged Britons to ignore “Brexiteers' madness” that it was too well-established to pull out of the country altogether.
"It is a disgrace that, more than two years after the result of the 2016 referendum, businesses are still unable to plan properly for the future,” Mr Enders said.
Theresa May could avoid taking Britain into the controversial Irish backstop if she changes her Brexit red-lines, the European Parliament is set to say.
A leaked draft of conclusions from a meeting of the body’s Brexit steering group seen by The Independent says tweaking the demands could see the UK “avoid deployment of the backstop”.
More from our Europe correspondent Jon Stone:
MPs calling for a second referendum have admitted they don't have the support in the Commons so they won't be tabling an amendment next week.
They threw down the gauntlet to Jeremy Corbyn, urging the Labour leader to back the plan.
Brexit questions are underway in the Commons, which will be followed by two urgent questions on free trade and the supervision of sex offenders.
TUC general secretary Frances O'Grady said they "did not get the guarantees we need" after meeting the PM in Downing Street this morning.
She said: "I think she needs to shift gear, stop playing to just the bad boys at the back of the class and listen to others for a change. We've been very clear with the PM that our priorities are protecting people's jobs and making sure workers' rights are protected now and into the future."
Ms O'Grady said the PM needed to offer a "sign of good faith" that she was serious about listening to other voices and she needs to ask for more time.
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