Mr Johnson is expected to secure the votes of most of the Brexit hardliners in the European Research Group of Tory backbenchers, as well as the group of former Conservatives expelled from the party for rebelling over no-deal.
But, with the Democratic Unionist Party opposing the deal, this will still leave him in need of the votes of around a dozen Labour MPs to get his motion over the line.
Johnson made a last-minute effort to lure Labour MPs with a set of concessions on workplace and environmental safeguards announced late on Friday night.
But the package – including a promise that ministers will report to parliament on whether the UK will match future EU employment regulations and a pledge not to open UK markets to countries with low environmental and animal welfare standards – was dismissed by Labour’s Laura Pidcock as an “empty gesture not worth the paper it’s written on”.
A handful of Labour MPs backed earlier versions of the agreement under Theresa May, and 19 signed a letter to the EU this month urging it not to delay Brexit any longer.
But it is far from clear that all this group will now support the Johnson package.
Ronnie Campbell, Sarah Champion and Melanie Onn – who all represent Leave-backing seats in the north of England – have indicated they may vote for the deal, and at least six others are expected to join them.
But in the joint letter, Mr Chilton and Mr Cortes urged them not to do so.
“Johnson’s deal is recognised by almost everyone as worse than any deal negotiated by Theresa May,” they wrote. “Ninety per cent of what he has secured is exactly the same as the deal secured by former prime minister Theresa May.
“Parliament rejected her deal three times and it is now being asked to vote on a bad deal for a fourth time.”
The letter accused Mr Johnson of “betraying” Northern Ireland by creating a customs border with mainland Britain.
And it said the deal would put workers’ rights, consumer and social protections as well as environmental safeguards “at risk” while limiting a future Labour government’s ability to back key industries.
“We the undersigned both campaigned on different sides during the referendum, but we both believe this deal to be so bad for our country, bad for our economy and bad for working people that we are uniting to call on you all to reject this deal,” said Mr Chilton and Mr Cortes. “It does not deliver for those who voted leave.
“Jeremy Corbyn is right to oppose this deal. All Labour MPs must join him. All of you were elected on a manifesto commitment to oppose ‘a damaging Tory Brexit’.
“We urge every Labour Member of Parliament to honour the manifesto commitment to which you pledged yourself before your constituents and your country in 2017 and oppose this truly damaging Tory Brexit.”
