With just more than six months until Britain leaves the EU, May has yet to reach a deal with Brussels on the terms of the exit. In the United Kingdom that means a vote by lawmakers.
And he set out his support for the option of a referendum if no general election is called - a policy delegates at the conference will vote on later on Tuesday.
Party members will be asked to agree that "if we can not get a general election, Labour must support all options remaining on the table, including campaigning for a public vote". "If they don't meet our six tests (for any Brexit deal), we will vote against it, and then we will take it from there", he said in a television interview.
This matters. With the views of pro-Brexit activists in Labour's heartlands in the North at complete odds with the views of Remain enthusiasts in metropolitan London and elsewhere, Mr Corbyn will have to bridge this divide when he makes his keynote leader's speech today. He declared: "We must learn to listen a bit more and shout a lot less".
The commitment was seemingly ad-libbed on stage by Starmer, as it was not included in the released text of his speech.
"Labour respects the decision of the British people in the referendum", he told the conference, adding: "But no one can respect the conduct of the government since that vote took place".
I will remind Mr Starmer that 5 million Labour voters also voted for Brexit.
His comments come the day before he will visit European Union chief Brexit negotiator Michel Barnier in Brussels.
Last week May issued an edict to Brussels when a summit of European Union leaders ended in a dismissal of British proposals. But EU officials say that amounts to unacceptable "cherry-picking" of elements of membership in the bloc without accepting all the costs and responsibilities.
"We are looking for a frictionless trade", May said in her speech at Bloomberg Global Business Forum 2018 held in NY.
"Of course there are big issues on the substance and how they will pay for these things". Look, every prime minister is faced with different challenges - domestically, globally.
"The Tory civil war on Europe that has been going on for years now risks our prosperity", he said.
Her wildly unpopular "Chequers Agreement" has pushed Brexiteers in her own party to the verge of mutiny.
May emerged from a lengthy meeting of her cabinet Monday, with what appears to be unity among her senior ministers about forging ahead with her Chequers Plan.
German Chancellor Angela Merkel warned Tuesday that time is tight.
Barnier himself tweeted on Wednesday that he was "working for an orderly Brexit" - Brussels-speak for averting a collapse of talks that could see Britain, and European Union businesses and citizens, plunged into legal limbo on Brexit Day next 29 March. "But if they can't and we end up in this position, then we have to reopen the question of whether we go forward with Brexit at all".