In a speech to his party's annual conference later, Starmer will say the Conservative government does not have a credible plan for Brexit, and that there is no majority in parliament for May's so-called "Chequers" proposals, which envisage close ties with the European Union in the trade of goods.
The motion was thrashed out by shadow Brexit secretary, Keir Starmer, along with more than 100 party delegates and is expected to pass on Tuesday.
Labour could play a deciding role in whether any Brexit deal is approved by parliament.
Len McCluskey, leader of the Unite trade union, a powerful Labour ally, said British voters had made a decision to leave the EU and "for us now to enter some kind of campaign that opens up that issue again I think would be wrong".
"The last one divided the country in a way we've not seen in recent years - I want a general election".
Speaking separately to the BBC's Peinaar's Politics yesterday, Len McLusky, the leader of Britain's biggest union Unite which is also Labour's leading donor, threw his support behind a second referendum, but said the option of staying in the EU should not be on the ballot, and rather the public should choose between whether to endorse May's final deal or opt for no deal at all.
Labour leader Jeremy Corbyn said over the weekend that he would shift his weight behind a decision to stage another Brexit referendum, should his party demonstrate clear support for the move.
"No "ifs", no "buts", Starmer will say in a speech to the party's annual conference, according to advance extracts. "That was clearly the mood of the room, and in fact that's reflected in the motion".
However, Labour's primary strategy appears to be forcing the government into an early general election, something the Brexit Secretary, Dominic Raab, categorially ruled out on Sunday. They did not do this to be offered a farcical referendum on no deal or a bad deal.
Mr McDonnell said: "The next leader of the Labour party has to be a woman". "And no one has a clue at the moment what is going to happen".
He claimed most Labour Party members and members of Mr McCluskey's union wanted the option of staying in the EU.
Confirmation that the party will vote against the deal in the Commons leaves Tory whips facing an uphill battle to persuade Eurosceptic MPs to back it.
"The leadership knows where the members are and they know where their voters are", the spokesman said.
But many Labour lawmakers think his old-fashioned socialism is a turnoff for the wider electorate. He has denied it and condemned anti-Semitism, but the furor has angered many Jewish party members and their supporters. "It will make the 2016 referendum look like a walk in the park".
Lewin added: "I wanted more for Remain Labour supporters and other grassroots campaigners, but we have a motion that effectively calls for Labour to support the EEA as a minimum, rejects the idea of a referendum exclusively on different forms of Brexit and increases the chances of a People's Vote".