Ross is the most senior Conservative to call for Johnson to go after the prime minister admitted for the first time that he had attended a party at 10 Downing Street in May 2020, when COVID-19 rules limited social gatherings to a bare minimum.

"Regrettably, I have to say his position is no longer tenable," Ross said. "I spoke to the prime minister this afternoon and I set out my reasons and I explained to him my position."

(Reporting by Andrew MacAskill; editing by William James)