Britain’s new finance minister Nadhim Zahawi told Boris Johnson to resign on Thursday, less than 48 hours after the prime minister promoted him to the job, saying the crisis engulfing the government would only get worse.

“This is not sustainable and it will only get worse, for you, for the Conservative Party and most importantly of all the country,” Zahawi said on Twitter after more than 50 ministers and aides resigned from the government.

“You must do the right thing and go now.”

Zahawi did not say he had resigned, but said he went to the prime minister’s office in Downing Street with cabinet colleagues on Wednesday evening to tell Johnson to “leave with dignity”.

“I am heartbroken that he hasn’t listened and that he is now undermining the incredible achievements of this government,” Zahawi said in a letter.

“The country deserves a government that is not only stable, but which acts with integrity. Prime Minister you know in your heart what the right thing to do is, and go now.”

However education minister Michelle Donelan resigned on Thursday less than 48 hours after she was appointed, saying it was the only way to force the hand of Prime Minister Boris Johnson to quit.

“I see no way that you can continue in post, but without a formal mechanism to remove you it seems that the only way that this is… possible is for those of us who remain in Cabinet to force your hand,” Donelan wrote in a resignation letter, saying she had “pleaded” with Johnson on Wednesday to resign.

“You have put us in an impossible situation… as someone who values integrity above all else, I have no choice.”