Prime Minister Theresa May has been pressed by union leaders to guarantee jobs and workers' rights after Brexit during a series of face-to-face meetings in Downing Street.
Len McCluskey, general secretary of Unite, stressed a no-deal Brexit would be "disastrous".
He hoped his first ever meeting with Theresa May was not a "PR stunt".
Mr McCluskey said Labour leader Jeremy Corbyn was right to refuse to meet the Prime Minister unless she ruled out a no-deal Brexit but urged Mrs May to extend Article 50 beyond March 29 for three months.
"I think the (Labour) amendment talks about nine months, I think that's way way too long, I'd like to see an amendment of about three months if proper negotiations are going to take place," he added.
TUC general secretary Frances O’Grady, Dave Prentis of Unison and Tim Roache of the GMB, also held separate meetings with ministers.
Ms O’Grady said she did not receive the guarantees she was seeking.
Workers are worried about their jobs and need reassurance about their future after the UK leaves the EU, she said.
"The Prime Minister should do the right thing and take a no-deal off the table so that genuine dialogue can take place.
"I was looking for guarantees on workers’ rights now and into the future," Ms O'Grady added.
The discussions are part of Mrs May’s bid to try and get widespread political backing in finding a Brexit agenda that would command a majority in the Commons after her plans were heavily rejected by MPs.
The Government move comes as there appeared to be growing support in Labour ranks for a parliamentary bid by former minister Yvette Cooper to extend Article 50, which would keep the UK in the EU longer, unless a deal is reached by the end of February.
Leading Brexiteers have attacked such initiatives, saying they would take control of events from the Government.
Despite Jeremy Corbyn branding the PM’s talks initiative a "stunt", a senior spokesman said the Labour leader’s request for the party’s MPs to boycott discussions with Mrs May did not extend to union leaders.
The Labour spokesman said: "As Jeremy set out last week, he is more than ready to engage in talks with the Prime Minister on the basis that no-deal is taken off the table.
"Unless she makes clear that she is prepared to move and compromise and accept the reality of the position, then she is simply continuing to try to run down the clock and prevent any solution to this crisis."
On the issue of a new national poll on EU withdrawal, shadow Brexit secretary Sir Keir Starmer said any referendum should have just two options.
Mr Starmer told ITV’s Peston: "I think there should definitely be Remain.
"And there should be a genuine Leave option.
"I think it would be better if it was a binary choice."
Meanwhile, the former Ukip leader, Nigel Farage insisted Mrs May’s withdrawal plans should be opposed, even if that risked a new referendum.
Mr Farage told ITV’s Peston: "Personally my view at the moment is better to vote down this dreadful deal and take the risk of a second referendum."
Ms Cooper’s Article 50 bid, which has cross-party backing including from Conservative Nick Boles, is one of a number of amendments that could be voted on next Tuesday if selected by Commons Speaker John Bercow.
A Downing Street spokesman said: "The Prime Minister 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."
Unions have been warning of the impact on jobs of a no-deal Brexit and have been pressing for assurances about employment rights after the UK leaves the EU.
Some have also argued in favour of a second referendum.