Labour's conference will vote later on Tuesday on keeping a second Brexit referendum as an option if May fails to get her Brexit plan through parliament, heaping pressure on a struggling prime minister.
His comments triggered an immediate backlash from Labour MPs who are pushing for a vote on Theresa May's Brexit deal.
In the published motion which the Labour conference will debate later on Tuesday, the party again set out is position on Brexit - it wants full participation in the EU's single market after Brexit and will reject a "no deal Brexit".
"If we are going to respect the referendum it will be about the deal", he said of any so-called People's Vote.
Many young members oppose it, carrying bags reading "Love Corbyn, Hate Brexit" at the gathering in Liverpool, while older, working-class Labour voters are in favour.
More than a 100 Labour constituency party offices have called for an conference vote on a second referendum and MPs, MEPs, union leaders and activists are also holding a march in support of a second vote.
Should Labour judge May's plans to have negative effect in response to the above questions, then the party is likely to vote down her Chequers plan in the commons.
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.
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The shadow Brexit secretary said Labour's analysis of the Chequers deal showed it would fail all six of the party's "Brexit tests".
A senior Labour MP dismissed it as "loony tunes" to think that the Prime Minister would call an early election, despite the party leadership throwing their weight behind one.
Mr Corbyn declined to say which way he would vote in any new in-or-out poll, while pointing out he backed Remain in the 2016 referendum.
"We weren't ruling out options and nobody was ruling out remain". Mr Corbyn confirmed there will be a vote on the party's Brexit stance during the four-day conference.
Mr Boulton replied: "We all know what the parliamentary arithmetic as it is, it's unlikely that, to put it crudely, that the turkeys will vote for Christmas, so why not just say, look if this can't be agreed by Parliament, let's put it back to the people, with crucially the option of remaining, if that's what people want.?" He has strong support among grassroots members, many of whom have joined since he was elected leader in 2015. But I'm not calling for a second referendum.
They did not do this to be offered a farcical referendum on No Deal or a Bad Deal.
"Anyone screaming "you are a racist" at Jeremy Corbyn has lost any sense of moral proportion and every shred of decency as well", he said to cheers from the Labour faithful.