EDINBURGH — Boris Johnson should resign after admitting he attended a party held in Downing Street during lockdown, Scottish Conservative leader Douglas Ross said.

As the leader of Johnson’s party in Scotland, Ross is the first senior colleague of the U.K. prime minister to directly call for his exit over the scandal.

Johnson apologized Wednesday after admitting he attended a drinks party in the garden of No. 10 Downing Street during Britain’s first lockdown — but claimed he thought the gathering of government staff was a “work event.”

Speaking to broadcasters in the wake of Johnson’s apology, Ross said the prime minister’s position “is no longer tenable.”

“I said yesterday that if the prime minister attended this gathering, party, or event in Downing Street on May 20 then he could not continue as prime minister,” Ross said. “So regretfully, I have to say that his position is no longer tenable.”

Ross, who also sits as an MP in the House of Commons, said he would write to the leader of the powerful 1922 committee of backbench Conservative MPs to make clear he no longer has confidence in Johnson. A vote on Johnson’s fate would take place among Conservative MPs if 15 percent of them express no confidence in this way.

“He is the prime minister — it is his government that put these rules in place and he has to be held to account for his actions,” Ross added.

The Scottish Tory leader was joined by several other Scottish colleagues in his call for Johnson to quit, including the party’s former leader Jackson Carlaw and its COVID recovery spokesperson Murdo Fraser.

Before he became Scottish Tory leader Ross was a close Johnson ally and was among those who originally backed his bid to become prime minister.

But Ross has sought to distance himself from Johnson — who polls suggest is unpopular in Scotland — since taking leadership of the Scottish Tories. He has criticized the U.K. leader for his approach to protecting the Union between Scotland and the U.K.

With polls continuing to show close to half of Scots back independence from the U.K, Ross’ decision to call for Johnson to resign puts the prime minister in a difficult place as he seeks to persuade Scots of the benefits of remaining in the Union.

“You can’t have a Scottish Tory leader saying ‘vote Conservative’ at a general election if they’ve called for their national leader to resign,” former No. 10 pollster James Johnson said.