In a stinging attack the former prime minister ambasted the “pretty poor” Brexit deal achieved by Mr Johnson, and praised Mr Sunak for trying to resolve Northern Ireland’s trade problems.
Sir John dismissed protests by Mr Johnson’s Brexiteer allies in the European Research Group (ERG) that the deal gives too much say to the European Court of Justices (ECJ), saying judges would have only a “tiny” role in trade disputes.
He also savaged the ERG’s claim that the new deal “undermines” UK democracy – arguing it would be a massive boost for democracy by paving the way for the return of the Northern Ireland Assembly, suspended over the DUP’s objections to the protocol.
“As far as trade is concerned the deal that Mr Johnson and Lord Frost [UK Brexit negotiator] did was pretty poor. After hailing it as a triumph and taking the plaudits for it, they were denouncing it within weeks while of course blaming the EU,” Sir John told BBC Radio 4’s Westminster Hour.
Making trade between Northern Ireland and the EU easier was vital, according to Sir John, who said the ERG’s insistence that the ECJ play no role was a “neuralgic point”.
Sir John said: “The fact that there will be a tiny, occasional involvement of the ECJ ought not to stop an agreement being made. They (the ERG) talk of democracy – that is thrown away when the Northern Ireland Assembly is not sitting.”
If the final text is agreed at talks this lunchtime, Mr Sunak is expected to hold a press conference with Ms Von der Leyen this afternoon before heading to the Commons to deliver a statement.
Tory Brexiteer Jacob Rees-Mogg put Mr Sunak on notice of a possible Tory revolt if the DUP does not support the deal – claiming that the support of both Mr Johnson and the DUP were vital.
He told GB News: “It will all depend on the DUP. If the DUP are against it, I think there will be quite a significant number of Conservatives who are unhappy.”
Mr Rees-Mogg also said that the position of Mr Johnson – whom he described as the “biggest figure in UK politics” – will also be “fundamental”.
Downing Street has not committed to giving MPs a vote on the final arrangements, but Tory Eurosceptic Theresa Villiers insisted that doing so is “crucial”.
The former Northern Ireland secretary told BBC Radio 4’s Today programme: “I can’t conceive of circumstances where something as significant as this could be finally agreed and implemented without MPs voting on it in parliament.”
Ms Villiers said she will consider the deal itself as well as talking to the DUP before deciding whether to support it but stressed the importance of restoring powersharing.
However, several senior Tory Brexiteers told The Independent they were minded to support the deal and expected Mr Sunak to sign the agreement without the backing of the DUP or ERG.
Former Brexit secretary David Davis told The Independent that his own “instinct” was to support a deal, and that he expected a rebellion of hardliners to be limited to a few dozen hardliners. “I think the idea of 100 rebels is absolute nonsense,” he said.
Senior Tory Martin Vickers, a member of the powerful 1922 Committee of backbenchers, added: “I was a staunch Brexit supporter, but we have to move on. On the basis of the outline [of the deal], I see no problem with it. We should rally round the prime minister.”
Deputy PM Dominic Raab said there had been a “paradigm shift”, revealing on Sunday that there had been “movement” with Brussels to address concerns around Northern Ireland not having a say on EU rules governing VAT and other areas.
The deal is also expected to emphasis a greater role for Northern Irish courts, as well as removing barriers to trade by introducing a “green lane” for goods moving between Britain and Northern Ireland.
But Mark Francois, chair of the ERG, said his group would still not back the deal unless it provided for EU law to be completely “expunged” in Northern Ireland.
Sir Keir Starmer – who has offered Labour support to get the deal through the Commons – said it is “almost inevitable” that the agreement will include some oversight by European judges. “The question will be whether the prime minister has got the strength to sell it to his backbenchers or not,” he said on Monday.
Alongside Sir John’s backing, fellow former Tory leader Michael Howard has urged MPs to support the Sunak deal. Writing in The Telegraph, Lord Howard he said it would “go a very long way to easing the practical difficulties” and “deserves the support of all Conservatives”.
Sir Malcolm Rifkind, a former Tory foreign secretary, also urged MPs to back what appeared to be a “pretty impressive” deal, warning that it was not worth listening to Mr Johnson – who has pushed the unilateral protocol bill as an alternative to the deal.
He told The Independent: “As Johnson was to a significant degree the cause of the problem by signing the protocol, I think he’s the last person who should be trying to damage what appears to be the pretty impressive efforts to get an alternative.”
Sir John played a key role in setting up talks that led to the Good Friday Agreement in 1998 credited with ending years of terrorism in Northern Ireland despite occasional outbreaks like the recent shooting of an off duty policeman.