Speculation over Ms Mirza’s husband

Her departure has sparked speculation that her husband Dougie Smith, a special adviser at No 10, will also walk out and that other key staffers could follow suit.

The resignation of one of his most loyal allies, who has worked with him for more than a decade, further weakens the Prime Minister as he battles to remain leader.

The Tory revolt against Mr Johnson over allegations of lockdown-breaching parties in No 10 was growing, with 13 Conservative MPs calling for his resignation.

Mr Johnson once praised Ms Mirza as a “brilliant thinker” and listed her as one of the five women who had influenced and inspired him the most.

Dominic Cummings, the former chief aide to No 10 who is agitating for the Prime Minister’s removal, said her resignation is an “unmistakable signal the bunker is collapsing”, adding that the “PM is finished”.

No 10 spokeswoman said: “We are very sorry Munira has left No 10 and are grateful for her service and contribution to Government.

“Andrew Griffith MP has been appointed Head of the No 10 Policy Unit and will work across government and with MPs as we continue to deliver and expand on our ambitious policy agenda.”