Boris Johnson is a “bloody nuisance” over a potential deal to end the Brexit deadlock with Brussels, it has been claimed.
The Government is increasingly hopeful of a breakthrough with the EU over the Northern Ireland Protocol, the post-withdrawal mechanism which prevents a hard border between the province and the Republic.
However, former Prime Minister Mr Johnson warned that dropping the Northern Ireland Protocol Bill - which would empower the UK to unilaterally scrap parts of its trading deal with the bloc - would be a "great mistake".
Mr Johnson's first intervention on Brexit since quitting No10 in September comes after Rishi Sunak launched a round of frantic diplomacy to secure a UK-EU agreement on fixing issues with the controversial post-Brexit trading arrangements.
Settling the row - and satisfying the Democratic Unionist Party - is key to restarting the Stormont Assembly.
After talks with EU leaders yesterday, Mr Sunak stressed a deal was "by no means done", amid mounting hopes a pact could be unveiled early this week.
The Government has indicated a successful outcome would mean the Protocol Bill would no longer be required.
A senior Government source said: "If we can find a way to satisfactorily resolve the issues with the Protocol then you wouldn't need the Bill. But we haven't resolved them yet."
Mr Johnson negotiated the Protocol, which created economic barriers on trade being shipped from Great Britain to Northern Ireland, as part of his Brexit deal, but later turned against it.
Problems with the Protocol, which prevents a hard border on the province’s 310-mile frontier with the Republic - the UK's only land boundary with the EU - have crippled politics in the province.
Critics say the mechanism has effectively drawn a trade border down the Irish Sea - hampering the exchange of goods between Northern Ireland and mainland Britain.
It has also fuelled the stalemate at Stormont, with the Northern Ireland Assembly not sitting.
The Protocol Bill is seen by Brexiteers as a key bargaining chip with the EU.
When Mr Sunak entered office, he paused the legislation as officials resumed intensive talks aimed at reaching a negotiated settlement.
A source close to Mr Johnson said: "His general thinking is that it would be a great mistake to drop the Northern Ireland Protocol Bill."
But a Government official told The Sunday Times: "Boris is being a bloody nuisance, winding up the DUP.
"He is causing mischief."
Critics suspect the ex-Premier of stirring up trouble so he can maintain support among hard right Tory backbenchers and the party grassroots if, as expected, the Conservatives lose the next election.
They believe it would give him the opportunity to seize back the leadership.
Mr Johnson’s intervention will raise concerns over a potential Tory rebellion if Mr Sunak's changes are put to a vote in Parliament.
Eurosceptic backbenchers will be angered by any compromise with the EU, particularly over the oversight role of the European Court of Justice in Northern Ireland.
Quizzed about the ex-PM’s intervention, Commons Leader Penny Mordaunt told Sky News’ Sophy Ridge on Sunday show: “Boris is being Boris”.
She hailed “encouraging signs” of a deal with Brussels, adding: “The PM has said that there’s still hard work to be done.
“Everyone will be pulling together to ensure that we are giving that deal the best chance possible.”
In a thinly-veiled warning to Mr Johnson, Ms Mordaunt went on: “What matters here is what the people of Northern Ireland think about the deal.
“This has to be acceptable to all communities in Northern Ireland … that’s the test, it's not what I or any other Member of the Commons thinks.”
Shadow Home Secretary Yvette Cooper said: “I hope that a deal is imminent. We have been urging the Government to act on this for a long time.
“We stand ready to support the Government, to support Rishi Sunak, in getting this through. We will provide political cover for him if he has problems with his own part on this.”