Four more Conservative MPs called for Boris Johnson to resign on Thursday over lockdown-breaking parties, as Downing Street sought to contain the political aftermath of the Sue Gray report.

The prime minister’s allies reiterated the defence that his attendance at a series of gatherings for departing staff was permitted as work duties. His official spokesman argued that Covid guidelines did not specifically rule out leaving drinks.

Boris Johnson during his statement on the Sue Gray report to the House of Commons on Wednesday.

He also formally apologised for misleading the media in repeatedly denying that any parties had taken place inside Downing Street during lockdown. He insisted this had been entirely inadvertent.

Nineteen Tory MPs have publicly called for Johnson to quit so far, well below the 54 required to prompt a confidence vote, but the trickle of new voices will unnerve Downing Street as backbenchers digest Gray’s description of rowdy and drunken gatherings inside No 10.

Along with those who have made public calls, two more have submitted and then withdrawn letters of no confidence, and at least three others have called for Johnson to resign but said they would not submit letters.

The former health minister Stephen Hammond was among those to say he had submitted a letter of no confidence. Hammond, who has a majority of 628 in his Wimbledon seat, described the conclusions of the Gray report as damning. “I cannot and will not defend the indefensible,” he said.

“I am struck by a number of my colleagues who were really concerned that it’s almost impossible for the PM to say I want to move on, as we cannot move on without regaining public trust and I am not sure that’s possible in the current situation.”

Two other MPs, David Simmonds and John Baron, said they had lost confidence in Johnson. A fourth, Angela Richardson, who quit as a parliamentary private secretary earlier in the year, said she would have resigned if she had been in Johnson’s position.