Lord Mandelson has accused Boris Johnson of trying to undermine Rishi Sunak by meddling in his attempts to strike a post-Brexit deal for Northern Ireland.
The Labour grandee and former Northern Ireland secretary was responding to a report that Johnson thinks it would be a 'great mistake' if Sunak's government overhauls his controversial Northern Ireland Protocol Bill as part of a new agreement with the European Union.
Mandelson said: "There's nothing that Boris is doing now, or indeed throughout our recent history with the European Union, that could possibly be described as helpful."
He suggested that the former PM is trying to "wreck" Sunak's attempts to come to an agreement because he is "opposed to the prime minister".
Watch: Boris Johnson warns against ditching Northern Ireland Protocol
Speaking to Sky News' Sophy Ridge on Sunday show, Mandelson said: “He and his supporters want to undermine the prime minister.
"It’s just a sort of continuation of the fratricidal war that we see in the Conservative Party.”
Mandelson, who was responsible for implementing the Good Friday agreement while Northern Ireland secretary, suggested a "period of silence" on Johnson's part would be welcome as Sunak tries to strike a deal.
The government has indicated that a new agreement with Brussels would mean Johnson's controversial Northern Ireland Protocol Bill would be overhauled.
A source close to Johnson suggested that the former PM thinks this would be a "great mistake" as the bill gives ministers powers to ignore EU rules or rip-up parts of the protocol.
The intervention has been interpreted by some as Johnson and his allies seizing an opportunity to attack Sunak's government.
The protocol was agreed in 2019 when Johnson was in power to ensure free movement of trade across the Irish border.
However, the DUP and other unionist groups are opposed to it, arguing different trade rules for Northern Ireland undermines its place in the UK.
Mandelson said that getting a new deal in place would build on the Belfast Good Friday Agreement, "reinforce the peace process" and prevent "any return to the violence that we've seen in Northern Ireland in previous decades".
However, Tory former defence secretary Penny Mordaunt said she didn't think Johnson sharing his views on Northern Ireland talks was an "entirely unhelpful intervention".
"I think the prime minister would give credit to his predecessors for enabling us to get this far," she told the BBC's Sunday With Laura Kuenssberg show.
"We have the bill, and the command paper that was produced at the time, and in part it is because of that that we are now able to have these negotiations and the EU is talking about things that it previously said it wouldn't talk about."
Mordaunt, who ran against Sunak for Conservative leader after Johnson's departure, said the bill acts as a reminder of the "bar" the EU has to get over in its negotiations with the UK.
She added: "I hope they do arrive at a deal but the deal has to work for all communities in Northern Ireland."