Boris Johnson has been told to keep his mouth shut as he faces accusations he is trying to oust Rishi Sunak.
The attention-seeking ex-PM threw a grenade into talks with Brussels on fixing his botched Brexit deal yesterday.
Senior Tories today warned Mr Johnson is plotting to bring down Mr Sunak so he can make a return to No10.
George Osborne said: “Boris Johnson is interested in becoming Prime Minister again… If you wait for Boris Johnson to do the sort of grown up sensible thing, he's not going to do it if he thinks it is a political opportunity to cause trouble.”
The former Chancellor, who was appearing on Channel 4’s Andrew Neil Show, added: “He wants to bring down Rishi Sunak and he will use any instrument to do it.
“If the Northern Ireland negotiations are that instrument, he will pick it up and hit Mr Sunak over the head with it.”
Officials believe the PM could reach an agreement with the EU on changes to the Brexit deal as soon as Tuesday.
The Government has been locked in negotiations for more than a year over complaints the arrangements for Northern Ireland are not working.
Mr Johnson at the weekend warned it would be a “great mistake” if Mr Sunak abandons legislation giving ministers the power to unilaterally overturn parts of the deal.
Quizzed about his intervention, Commons Leader Penny Mordaunt said “Boris is being Boris” as she insisted his comments were not “completely unhelpful”.
But in a thinly-veiled rebuke to Mr Johnson, she told Sky News: “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.”
Former Northern Ireland Secretary Lord Mandelson said: "There's nothing that Boris Johnson is doing now or, indeed, throughout our recent history with the European Union that could possibly be described as helpful.
"He's wrecking, he's trying to wreck the thing because he's opposed to the Prime Minister.
"He wants and his supporters want to undermine the Prime Minister."
The Labour peer said he hoped Mr Johnson would remove "himself entirely from this matter", adding: "A period of silence on his part I think would be welcome."
Lord Hain, another Labour former Northern Ireland Secretary, said Mr Johnson was "mischief-making" after he "created this mess" and "failed to resolve the issues" over a number of years.
Tory Brexit wars explainer
Why are we still talking about Brexit?
Boris Johnson promised voters an “oven ready” Brexit deal at the 2019 election, but the arrangements for Northern Ireland have led to continued rows. The UK and EU have been locked in talks on how to fix them for over a year, but it is hoped an agreement could finally be sealed this week.
What is the Northern Ireland protocol?
As part of the Brexit negotiations, the two sides agreed the Protocol, which was designed to avoid the need for a border on the island. But this has led to disruption on goods crossing the Irish Sea, with new checks imposed on those moving to the province.
Who is unhappy?
Concerns have been raised by unionists that Northern Ireland's place within the UK is being undermined, with the Democratic Unionist Party blocking the formation of a powersharing executive at Stormont until the issue is resolved.
What does Boris Johnson want?
As PM he drew up legislation that would give ministers the power to unilaterally scrap parts of the Protocol. He has urged Rishi Sunak not to abandon the law even if he gets an agreement with Brussels on changes.
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