Dominic Cummings has predicted the UK is "in for a nightmare" because Prime Minister Boris Johnson “doesn’t think this is over” despite announcing his resignation.
Mr Johnson has stepped down as Tory leader, making the announcement outside No.10 on Thursday afternoon.
More than 50 politicians tendered their resignations in the last few days, including former Chancellor Rishi Sunak and ex-Health Secretary Sajid Javid.
But despite Mr Johnson’s resignation, Mr Cummings said he believes the PM “doesn’t think it’s over”.
Dominic Cummings
Aaron Chown
Boris Johnson announced his resignation earlier today
HENRY NICHOLLS
Writing on Twitter, the PM’s former aide said: “I know that guy and I'm telling you – he doesn't think it's over.
“He's thinking 'there's a war, weird s*** happens in a war, play for time play for time, I can still get out of this, I got a mandate, members love me, get to September.”
Mr Cummings also warned of “carnage” with Mr Johnson staying on as PM until a permanent successor is confirmed.
He added: “If MPs leave him in situ there'll be CARNAGE.”
In a separate tweet after the announcement, Mr Cummings added: "In character. Blames everyone else.
"Thinks he's the real victim.
"We're all in for a nightmare if he's allowed to squat."
Announcing that he has stepped down, Mr Johnson said: "It is clearly now the will of the Parliamentary Conservative Party that there should be a new leader of that party and therefore a new Prime Minister.
"And I have agreed with Sir Graham Brady, the chairman of our backbench MPs that the process of choosing that new leader should begin now.
"The timetable will be announced next week and I have today appointed a Cabinet to serve, as I will, until a new leader is in place."
The PM said he had tried to persuade his Cabinet it would be “eccentric” to change Prime Minister now but “I regret not to have been successful in those arguments”.
After acknowledging that "in politics, no one is remotely indispensable", Mr Johnson told the British public: “I want you to know how sad I am to be giving up the best job in the world."