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Some MPs appeared to have been waiting for the prime minister’s appearance before the Commons liaison committee on Wednesday before making their minds up on the controversy surrounding his top adviser.
But Johnson’s appeal for the country to “move on” left them unconvinced.
Ex-ministers George Freeman and Andrew Selous, as well as Pauline Latham and David Simmonds, have added to calls for the top aide to go.
The BBC reported that Henry Smith had also urged Cummings to quit.
It is now believed there are more than 40 Tory MPs united against the PM’s chief adviser.
Johnson has also suffered the resignation of Scotland minister Douglas Ross over Cummings’ apparent breach of the coronavirus lockdown by driving 260 miles from London to Durham to self isolate with his family, and then later taking a further drive to Barnard Castle, where the family walked on his wife’s birthday.
Freeman said it has become clear from constituents’ letters that nothing has matched “the scale and depth of the anger” over this in his 10 years as an MP.
The Mid Norfolk MP said his “credibility and patience was stretched” by Cummings’ account of his trip to Barnard Castle on a bank holiday weekend “when the whole nation was being instructed to stay at home”.
“It’s clear to me that Mr Cummings’ actions have seriously undermined the authority of the government, the prime minister and the public health message,” he said.
“By continuing in office, Mr Cummings is now regrettably undermining the government, party and prime minister he is employed to serve.
“For all these reasons, I believe Mr Cummings must now apologise and resign his post.”
Selous said Cummings’ behaviour has “damaged the prime minister, the government and most critically of all, the integrity of our vital public health messages.
“I therefore think he should no longer have a role in government.”
Latham said: “We should all follow that guidance no matter who we are. That is more important than the role of any one government adviser and I cannot defend the indefensible.”
Johnson on Wednesday afternoon refused to publish evidence Cummings presented to him to save his job, and rejected calls for a Cabinet Office inquiry.
The PM also denied there was a conflict between how Cummings interpreted the rules compared to parents who stayed at home during the lockdown despite having the virus or its symptoms.
It came as a YouGov survey for The Times showed the Conservative lead over Labour dropping by nine points during the Cummings saga.
Support for the Tories fell four points in the week to Tuesday, to 44%, with Labour rising five points to 38%.