Keir Starmer has been forced to reaffirm that Labour has not ruled out a referendum that could cancel Brexit, after John McDonnell appeared to insist the party could only back a vote on the deal itself.

Labour has said it intends to vote against Theresa May’s deal, if it resembles the Chequers proposals set out in the summer white paper. However it is highly uncertain what would happen next if MPs reject her agreement.

Remain campaigners are urging the party to take that opportunity to seek to reverse Brexit altogether, but the frontbench would much prefer a general election.

Keir Starmer clashed with Corbyn on Brexit 'to brink of resignation' Read more

The shadow Brexit secretary was involved in a five-hour late-night “compositing” meeting on Sunday night, that thrashed out a statement committing the party to seeking a people’s vote if it cannot get a general election. The key sentence said: “If we cannot get a general election Labour must support all options remaining on the table, including campaigning for a public vote.”

But the shadow chancellor infuriated anti-Brexit campaigners on Monday morning by echoing remarks made on Sunday by Len McCluskey, the Unite general secretary, suggesting Labour should not accept any referendum that appeared to question the result in 2016.

“We’re respecting the referendum. We want a general election. If we can’t get that, we will have a people’s vote. The people’s vote will be on the deal itself, and whether we can negotiate a better deal,” McDonnell told the BBC.

Starmer said the compromise reached at the Sunday night meeting – and which will be ratified by conference delegates on Tuesday – deliberately left open what the question would be. Grassroots campaign group Momentum is advising its members to vote for the motion.

“The meeting was very careful not to define the question, because we don’t know if we’re dealing with a deal or no deal – but equally, it was not ruling anything out, and the words ‘on the deal’ were removed, to emphasise the openness of that question,” he said.

“Certainly, there was no ruling out of anything, and no ruling out of remain as an option. That was clearly the mood of the room, and in fact that’s reflected in the motion.”

McDonnell later sought to clarify his remarks, saying: “Keir is right. We are keeping all the options on the table.” But he has repeatedly stressed the risks of a new vote, saying it could embolden the far right.

Brexit has pitted the Labour membership, which is overwhelmingly pro-remain, against the instincts of the leadership, who fear the impact on the party’s support in leave-voting seats if it appears to be seeking to overturn the referendum.

The issue was the overwhelming favourite for discussion among local parties and trade unions – and Labour’s new position closely resembles that of the TUC, ratified at its own congress earlier this month.

Starmer will use his speech in the Brexit debate to formally set out Labour’s objections to the Chequers position – based on the “six tests” he has set for assessing any potential deal with Brussels.

One of the tests is that any deal must maintain exactly the same benefits as the single market and customs union – making it very difficult for May to meet.

Starmer will say, “let me be very clear – right here, right now: If Theresa May brings back a deal that fails our tests – and that looks increasingly likely – Labour will vote against it. No ifs, no buts”.

He will also make clear for the first time that Labour will not back a “blindfold Brexit”, in which the details of Britain’s future relationship with the EU are left until after March next year to be negotiated.

“If the prime minister thinks we’ll wave through a vague deal asking us to jump blindfolded into the unknown she can think again. You can’t meet Labour’s tests by failing to provide answers. We will vote down a blind Brexit,” he will say.

He will add, “the party that once promised to fix the roof while the sun was shining now seems intent on burning the whole house down.”

When Labour first announced its tests, the prime minister said, “I have been looking at those tests because, actually, there are principles that the Government have, time and time again, said we are determined to meet.”

The Conservatives seized on Labour’s potential support for a people’s vote, with the prime minister tweeting that the party wanted to “take Britain back to square one – betraying all those who voted in the 2016 EU referendum”.