Len McCluskey has warned Labour MPs they must vote against Theresa May’s Brexit deal.
Writing for HuffPost UK on Monday, the general secretary of the Unite union said the Commons must not be “bounced into a bad Brexit” by the prime minister.
He said MPs must “hold their nerve, act for the people and vote down her plan”.
Some Labour MPs have indicated they would be willing to back the government’s Brexit deal in order to avoid the UK leaving the EU with no deal at all.
Jeremy Corbyn is due to set out his plan for a “good Brexit” at the CBI conference on Monday.
May will use her own speech at the conference to face down rebels in her own party who want last minute changes made to her deal.
The prime minister will say the withdrawal agreement has been “agreed in full”.
Andrea Leadsom, the pro-Brexit Commons leader, has said there is “still the potential to improve” the draft.
She is reported to be one of five ministers hoping to change the deal before it is rubber stamped at an EU summit on Sunday.
Michel Barnier, the EU’s chief negotiator, said this morning the deal was “fair and balanced”.
After a week which saw Cabinet ministers resign over the Brexit deal and the campaign to replace May broke cover, all eyes will be on Sir Graham Brady, chairman of the backbench 1922 Committee.
Speculation is mounting over whether he has received the 48 letters of no confidence required to challenge her leadership of the Conservative Party.
Conservative MP Anne Marie Morris said this morning there was “no question” that the threshold would be reached this week.
But environment minister Therese Coffey said if there was to be a vote of no confidence May would “win it convincingly”.
May is expected to say today there is “an intense week of negotiations ahead of us”.
“During that time I expect us to hammer out the full and final details of the framework that will underpin our future relationship and I am confident that we can strike a deal at the council that I can take back to the House of Commons,” she will say.
“The core elements of that deal are already in place. The Withdrawal Agreement has been agreed in full, subject of course to final agreement being reached on the future framework.”
In his blog, McCluskey said: “If the Prime Minister would only stop listening to the hard-line rump on her own backbenches, she would realise that there is a different deal available.
“That it is perfectly possible to secure a natural majority in parliament for leaving the European Union, as the British people mandated in June 2016 but in a way that unites the nation.
“She needs to reach out to Jeremy Corbyn and forget about Jacob Rees-Mogg and her disloyal cabinet.
“She needs to actually listen to trade union concerns, rather than pretending to do so. Jeremy Corbyn extended the possibility of agreement in his Party Conference speech in September. Now is the time to take up the offer.”
But he added: “I doubt Mrs May will take up the path offered by Corbyn seeking what is right for our nations - MPs should, therefore, hold their nerve, act for the people and vote down her plan.”
Former president of the European Council Herman Van Rompuy said there was little prospect of altering the Brexit deal at this stage.
He told the BBC: “On the main parameters what I hear is that there is almost no room for manoeuvre.”
It was reported on Monday that the EU could extend the transition period by two years in order to allow more time for a trade deal to be negotiated.
According to the Financial Times, the EU’s chief Brexit negotiator, Michel Barnier, told ambassadors the transition could go on until as late as December 2022.