Boris Johnson would be a "permanent nightmare" for whoever succeeds him in Downing Street and will want "revenge", a former Tory leader has said.
William Hague said the outgoing Prime Minister "lives his life as a performance" and he was likely to be a thorn in the side of either Rishi Sunak or Liz Truss in future.
The pair are slugging it out to become the next PM in the September in an increasingly bitter race to win the support of Tory members.
But Mr Johnson looms large over the contest, with rumours swirling wildly about whether he plans to mount a comeback.
Tory donor Lord Cruddas, who is spearheading a grassroots push to give members a vote on the PM's resignation, said Mr Johnson had told him over lunch last week that he wants to fight the next election as Tory leader.
Writing in the Times, Lord Hague said: "He is going to be Heath with jokes added in, and Thatcher with consistency taken out, all rolled into a bundle of resentment, denial, attention-seeking and attempted vindication that will be a permanent nightmare for the new Prime Minister.
"That he wants revenge on Rishi Sunak is already apparent, but if Liz Truss is elected, she will face the identical problem.
"The chances of her loyalty to him being repaid are close to zero.
"Boris lives his life as a performance, and he will want the next act to fill every seat in the theatre of British political life."
Lord Hague joked about how he struggled to deal with John Major, Margaret Thatcher and Ted Heath when he was Tory leader and said Mr Sunak or Ms Truss would soon face the same problem.
"Johnson might be near the bottom of the class of PMs in the reputation with which he leaves office, but he could be top of the class in the trouble he can cause afterwards," he said.
It comes as Tory grandees pleaded with both campaigns to stop the fierce personal attacks - and warned the acrimony could cost the party the next election.
Lord Fowler, who served under Margaret Thatcher and is now a crossbench peer, said the contest had been "extraordinarily divisive" and risked the Tories becoming the "nasty party" again.
“If I was to advise the Conservative Party to remember one thing it’s that a divided party simply doesn’t get elected," he said.
Lord Lamont of Lerwick, a former chancellor who endorsed Mr Sunak, said: “They both ought to tell their followers to calm down a bit.
"I think it’s trivial and silly and is damaging the party."
Conservative members will begin to receive their ballots next week for the final round of the contest.
The next Prime Minister will be revealed on September 5.
