The Prime Minister has been under pressure in recent weeks over parties held in and around Downing Street during lockdown, one of which was to celebrate his birthday, which is believed to have involved a cake.
While urging Mr Johnson to quit, Mr Blackford said: “The impending National Insurance tax hike hangs like a guillotine while they eat cake.”
In response, the Prime Minister said: “I don’t know … who has been eating more cake.”
Kirsten Oswald, Mr Blackford’s deputy leader at Westminster, hit out at Mr Johnson’s comments.
“Boris Johnson has proven, yet again, that he is completely unfit for office,” she said.
“Whether it’s his body shaming jokes, his racism, his homophobia or his wider misconduct in public office, Mr Johnson is giving people another telling insight into his odious character.
“The Prime Minister’s frequently offensive remarks will have a damaging impact on many young people and need to be called out.
“He is sending an appalling message to society, that the most powerful person in the UK Government thinks it’s OK to mock people because of their bodies, race, sexual orientation and religion.
“The longer Tory MPs allow Mr Johnson to cling on to power, the more damage they will do to any remaining public trust in this discredited Government.”
Eating disorder charity Beat also criticised Mr Johnson’s comment to Mr Blackford.
Its director of external affairs Tom Quinn said: “It is completely unacceptable that the Prime Minister should resort to making fatphobic comments.
“It shows a clear lack of regard for, or understanding of, the 1.25 million people in the UK affected by eating disorders, and we are extremely disappointed that he has deemed this to be an appropriate response.
“We would urge the Prime Minister to educate himself on the seriousness of these mental illnesses, and limit his comments of other MPs to their actions, not their appearances.”
Mr Johnson had been pressed during PMQs on whether he would publish the full report by senior civil servant Sue Gray surrounding parties held at Downing Street during the Covid pandemic.