Labour’s pro-Remain MPs have issued a stark warning to any colleagues thinking of backing a Boris Johnson deal that they risk destroying the party and ruining its chances at the next general election.

The prime minister’s chances of securing a majority for any new deal with Brussels when parliament meets next Saturday are on a knife-edge, and depend on how many pro-Leave Labour MPs defy the party whip to back the government.

Earlier this month 19 Labour MPs wrote to the European Commission president Jean-Claude Juncker saying they “wish to see the British EU referendum result honoured without further delay”. This was despite Jeremy Corbyn saying he believed that no Labour MP could back a Johnson deal.

On Saturday Labour’s Brexit spokesman, Keir Starmer, said the party would push for a second referendum on any deal tabled by Johnson and would do everything in its power to avoid a no-deal outcome. Starmer has made clear he would campaign to Remain in a second referendum.

The 19 MPs are now coming under intense pressure both from party whips and colleagues not to help Johnson get a deal across the line – something many believe would allow him to enter a general election having defused the threat from the Brexit party and in a strong position to form a majority.

Frontbencher Clive Lewis said Labour MPs who backed Johnson would not only be foisting a deal on the country that was bad for jobs and for Britain but would also risk consigning themselves and their party to election defeat.

“I would urge any Labour MP who is considering voting for a Tory deal to think very, very carefully,” Lewis said.

“If Boris Johnson can go into an election as the man who delivered Brexit, we will be in serious trouble. It will be the Tories, not supportive Labour MPs, that reap the electoral rewards of any such deal. They will in effect be signing up to their own political demise.

“To top it, the communities those MPs came into politics to represent will be the ones who suffer the consequences. I know, in their hearts, many of them know this to be true. I’d really urge them to reflect on that and act accordingly.”

Owen Smith, the former leadership challenger, added: “Any Labour colleague who votes for Johnson’s Brexit will be betraying our values and our country. He is proposing the hardest of Brexits for Britain, with no guarantees for workers’ rights or environmental standards while Northern Ireland gets cast into some limbo-land, neither truly in nor out, but with the constitutional question recklessly returned to the heart of Irish politics.

“For this to go through on the back of any Labour votes would be a moment of great shame for our party. The only way a vote for this deal could ever be justified is if it has a confirmatory referendum attached.”

Several of those considering backing a deal presented by the government say they are still keeping their options open and are waiting to see what precisely it is that Johnson returns with from Brussels, if anything. Stephen Kinnock said he would make his decision based on what safeguards there were on issues such as workers’ rights.

“Our number one aim is to protect peace and security on the island of Ireland, and it’s clear that the Irish government and the Northern Irish communities are best placed to judge whether the new deal will achieve that aim,” Kinnock said.

“The other key issue is the future relationship, and on this point it’s vital that the government provides guaranteed safeguards on workers’ rights, environmental and consumer standards, and that parliament will be given binding votes at key milestones throughout the future relationship negotiations [which will be taking place during the transition period].”

Gloria de Piero, who is stepping down at the next election and was one of the 19 who signed the letter to Juncker, said she would take a decision based on the commitments she made in her last election manifesto to honour the 2016 referendum result, stop no deal, and leave with a deal. “I will look at the deal very carefully,” she said.

Anna Turley said she could not understand how any Labour colleagues could be considering backing a Johnson deal which would leave the people Labour exists to represent poorer. “I just really struggle to see how voting for a Boris Johnson deal can be supported by any Labour MP. We know it will damage the economy. We know it will hit the poorest hardest. We know it will increase inequality in this country and put our precious NHS at risk. We know it makes us more insular, and peace in Northern Ireland more vulnerable. None of these things are in keeping with the values of our movement.”