Keir Starmer was forced to confirm that labour did not rule out a referendum that could cancel Brexit, after John McDonnell apparently insisted that the party could only support 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.

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.

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”.