Dominic Cummings has claimed Boris Johnson is supporting "human hand grenade" Liz Truss as he thinks "she'll blow and he can make a comeback".
The embittered ex-No10 aide said he'd given Ms Truss the cruel moniker back when they both worked at the Department for Education as she "blows up all she touches".
Mr Cummings claimed the PM - who he nicknamed the "shopping trolley" due to his repeated U-turns - was backing her as he wants another chance at power.
Ex-Chancellor Rishi Sunak and Ms Truss will go head to head over the summer in a Tory members vote, which will name the new Prime Minister at the start of September.
Mr Johnson insisted on clinging onto power until a new leader was elected, but the demob-happy PM has so far hosted a lavish leaving do at Chequers and tried out a typhoon jet.
However, in his last PMQs he dropped hints that he is already plotting a comeback by telling MP: "Hasta la vista, baby".
Mr Cummings tweeted: "Why is the [trolley emoji - meaning Boris Johnson ] supporting the supporting the human hand grenade [ Liz Truss ]?
"Cos [trolley emoji] knows she's mad & thinks she'll blow & he can make a comeback.
"Why did I give her this nickname? Cos she blows up all she touches."
The ex-aide has gone to war with Mr Johnson after he left No10 in November 2020 following a power struggle with the PM's wife Carrie.
There has been speculation that he backs Mr Sunak after he has conspicuously failed to criticise him - while slating everyone else.
It comes as Mr Johnson told the Commons it was "mission largely accomplished - for now" raising suspicions he would like another tilt at the top job.
One friend of the PM told the Mirror that he "really thinks he will be back" while another said he was "deluded" but confirmed he wanted to return to No 10.
Mr Johnson's political hero, Winston Churchill, served a second term in office six years after he was ousted following the Second World War.
At PMQs, Mr Johnson's allusion to the Terminator movie reminded MPs of the other well-known quote from the movie: "I'll be back".
He was given a standing ovation from the Tory benches but his predecessor Theresa May notably refused to clap, and only appeared to stand reluctantly.
In his advice to his successor - and a dig at Tory MPs who plotted to oust him - Mr Johnson said: "Focus on the road ahead, but always remember to check the rear-view mirror."
The PM also took a swipe at Rishi Sunak - who his allies believe triggered his downfall by quitting as Chancellor, leading to dozens of other ministerial resignations.
He said big infrastructure projects would not have been built "if we'd always listened to the Treasury" in a dig at his former Chancellor.
He gave some "words of advice" to his successor which included: "Cut taxes and deregulate wherever you can and make this the greatest place to live and invest, which it is.
"I love the Treasury but remember that if we'd always listened to the Treasury we wouldn't have built the M25 or the Channel Tunnel."
Mr Johnson's allies have backed Ms Truss, who has promised vast tax cuts, which Tory sources claim fiscally conservative Mr Sunak had blocked.
