George Soros-funded anti-Brexit lobby group Best for Britain have denounced Labour backing for a second referendum just hours after praising the policy as a “welcome step” following John McDonnell’s announcement that an option to remain in the EU would not be on the ballot.

The campaign said it was unacceptable that the prospect of remaining in the European Union (EU) would not be on the table in the so-called ‘people’s vote’ outlined by the shadow chancellor on Sunday evening, in press releases published to Twitter.

“John McDonnell has confirmed that Labour would back a public vote on Brexit if a general election doesn’t take place,” Best for Britain boss Elaine Todd said.

“It would be unthinkable for this to not include an option to stay in the EU, and not what the 90 per cent of Labour members who back a public vote would expect.”

The statement came just hours after the Soros-backed group issued one describing the announcement Labour would back a second vote as “a welcome step forward for the party”.

With delegates set to debate on Tuesday whether “Labour must support all options … including a public vote” if lawmakers reject the government’s Brexit deal, Remain campaigners were angered that the motion — which took five hours to develop at a meeting Sunday — did not explicitly specify that a new referendum should include the option to overturn more than 17.4 million votes to leave the bloc.

According to the Associated Press, the shadow chancellor said that “we argued for ‘remain’ in the past but we lost that vote so we have to respect that”.

Asked what the vote would be on, Mr McDonnell told the BBC that “Parliament will decide what will be on that ballot paper, adding he would be “arguing that it should be a vote on the deal itself.”

But other Labour figures were unhappy about the desire of the party leadership to see the referendum result honoured including David Lammy, who described Britain’s decision in 2016 as a “car crash” that would “wreck the country”.

In a statement, the MP for Tottenham said he was “happy” that the party was “moving towards a People’s Vote”, but urged campaigners to “turn up the noise” over the next few months in demanding a chance to overturn the historic referendum result.