Melanie Onn has become the latest Labour MP to declare she is prepared to vote for Boris Johnson’s Brexit deal despite intense pressure to oppose it.

Onn, who represents the leave-voting seat of Grimsby, has written an article for the Guardian with the Conservative MP Victoria Prentis urging colleagues to “use this unique chance to help us move on and get back to helping our constituents”.

They wrote: “The risk of letting this final shot at a deal slip through our fingers is too great. Our collective hope rests on brave Labour MPs, and indeed others, who can see that.”

Onn, who comes from a working-class background and grew up in Grimsby, was considered a rising star in the party before she resigned from a frontbench role in March in protest at Labour’s support for a second referendum.

The Democratic Unionist party opposes Johnson’s deal and is encouraging Conservative MPs to vote against it, meaning the prime minister is likely to need to win over more Labour MPs than the handful of ardent Brexiters who supported Theresa May’s deal.

The Labour leadership have said Johnson’s deal shifts the “level playing field” pledges on workers’ rights and environmental standards from the legally binding withdrawal agreement to the political declaration. The shadow chancellor, John McDonnell, said on Friday he did not expect more than a few Labour MPs to be won over.

He told BBC Radio 4’s Today programme: “I don’t believe there are Labour MPs out there who will not think through the detail of this deal and then, when they see the impact – particularly on environmental, consumers’ and workers’ rights – will vote for it.”

Among the Labour MPs who have said they will vote for the deal are Jim Fitzpatrick, who represents Poplar and Limehouse in London, and Ronnie Campbell, the MP for Blyth Valley in the north-east.

Asked about Campbell’s views, McDonnell said: “I’m going to have a chat with Ronnie because he is a good socialist and all his life he has fought alongside me to protect trade union rights and develop trade union rights.

“On this one I’m going to have a chat with him and point out to him: please do not give this power to Boris Johnson, because you know what he’ll do. He’ll undermine trade union rights. Boris Johnson and those Tory MPs that populate the cabinet, these are the extreme right who have attacked trade unions throughout their political careers.”

He added: “No MP, as far as I’m concerned, who has the true interest of their constituents at heart can allow that to happen.”

Several more Labour MPs who in the past have expressed their preference for a Brexit deal over a second referendum will continue considering their decision overnight.

One said colleagues were finding the decision very difficult, with more than one in tears at a recent meeting. They suggested trades unions were ringing round MPs saying they would not receive financial support in future if they voted with the government.

Many of the “MPs for a deal” group, which includes Onn and Prentis, are likely to support the Letwin amendment, which effectively delays the final decision. They would then hope to amend the withdrawal agreement, potentially to allow more reassurances on workers’ rights and other issues.

Jon Lansman, the chair of pro-Jeremy Corbyn campaign group Momentum, suggested the party’s ruling national executive committee could replace Labour MPs who defied the whip.

Jon Lansman (@jonlansman) Johnson's deal will be a wrecking ball through the lives & well-being of ordinary people across Britain. Labour MPs cannot and must not vote for it. If they do, the NEC will have no choice but to replace them with a new, socialist Labour candidate at the next election

Corbyn himself has hinted he is minded to take a gentler approach, saying: “I believe in the power of persuasion rather than the power of threat.”