Penny Mordaunt’s rivals in a future Conservative leadership contest have been blamed for briefing coup rumours meant to ruin her chances of eventually taking over from Rishi Sunak, the Guardian has been told.

The Commons leader, who came a close third in the leadership contest won by Liz Truss in 2022, has in recent days been at the centre of speculation over an alleged plot to try to oust the prime minister to avert a general election disaster for the Conservatives.

But her allies insisted on Monday that it was “nonsense” that she had been talking to Tory rightwingers as part of any plot. Everyone knows that Penny’s rivals are just trying to stir up trouble,” one supporter said.

“She’s loyal to Rishi and doesn’t want to add legitimacy to all this fluff by publicly denying the claims.”

On a visit to the Midlands on Monday, Sunak said he was “not interested in Westminster politics” and insisted that his party was “united” as he tried to quell speculation that he could become the latest Tory leader to lose his grip on power.

Those loyal to Sunak say they do not believe Mordaunt herself is the source of rumours that she might stand against him.

“I don’t know who is putting it about but I don’t think it is her,” said one supportive cabinet minister. “My best guess is those who are likely to stand against her at a leadership election in the future are trying to spoil her candidacy now by making her look disloyal.”

Senior figures from across the party came to Mordaunt’s defence, with one former cabinet minister from the Boris Johnson era suggesting that the speculation was a result of a “three-bottle lunch” between supporters of her rivals and members of the press.

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The reports about a potential Mordaunt coronation emerged on Friday evening in the Daily Mail and Daily Telegraph, which said a meeting had taken place between leading figures on the Tory right and prominent supporters of Mordaunt’s last leadership bid.

The former cabinet minister said: “There’s a lot of unhappiness in the party towards Rishi and the No 10 operation. But we don’t know whether there briefing is coming from a 12-year-old junior spad in a Whitehall department or a cabinet minister.”

One minister from the centre of the party said the claims were “pretty unfair” to Mordaunt, who they said had been doing the constituency party dinner circuit for years.

A senior party figure on the One Nation wing said Mordaunt was being “used” by supporters of her rightwing rivals for the Tory crown – including Kemi Badenoch, Suella Braverman and Simon Clarke – to launch a leadership contest.

“They’re using her to try to take over from Rishi. None of the rightwingers can get the centre of the party onboard. Even if there’s a coronation, Penny still loses the election and it takes her out of play for the longer term,” the MP said.

A leading rightwing MP said it was unlikely their colleagues would back Mordaunt, even as a compromise candidate, because of her liberal position on social issues such as trans rights. “I don’t sense any appetite for trouble-making though,” they said. “I believe this is just Penny’s rivals trying to do her down.”

While Sunak’s supporters have dismissed the idea that electing a fourth party leader within a single parliament could revive the party’s fortunes, many remain unhappy about the way in which his Downing Street operation has worked in recent weeks.

Even the prime minister’s allies say he was slow to remove the whip from the former deputy chair Lee Anderson for his comments about the London mayor, Sadiq Khan, and then indecisive once more over what to do about the Tory donor Frank Hester.

Many say they are frustrated by Downing Street’s inability to stick to a single message.

“The national insurance cuts in the budget had the potential to cut through,” said the cabinet minister. “But it won’t do that if we don’t keep talking about it. Just because we haven’t seen an eight-point narrowing in the polls in the week after the budget doesn’t mean we should stop talking about it.”

Allies say Sunak has been frustrated by the latest bout of infighting and he has no intention of calling an early election to fend it off, as has been suggested by some in the party. “The plan is still to do it in the last quarter of the year,” said one.