In a U-turn on his call for the prime minister to quit, Mr Ross said "the middle of an international crisis is not the time to be discussing resignations, unless it's the removal from office of Vladimir Putin".

The decision comes little more than a week before the Scottish Conservatives conference in Aberdeen, which Mr Johnson has been invited to address.

With most Scottish Conservative MSPs having joined Mr Ross in calling for the PM to resign, it had been expected that Mr Johnson would address the conference via video.

However, the PM could now reportedly attend in person in light of Russia's invasion of Ukraine.

Mr Ross had called on Mr Johnson to go following the PM's admission that he attended a "bring your own booze" event at 10 Downing Street at the height of the UK's first national COVID lockdown.

The 20 May event is among 12 alleged COVID rule-breaking gatherings in Downing Street and Whitehall held during periods of strict coronavirus measures.

But Mr Ross, both an MP and MSP, has now said there should be a "pause" on the partygate row as Russia continues its assault on Ukraine.

"There will be a time and place to debate partygate but, as even [Labour leader] Keir Starmer said at the weekend, we should put that on pause while there is war in Europe," the Scottish Tory leader said.

"It's essential that we all fully support what the UK government is doing.

"In light of Russia's appalling actions, the government and PM need our backing, and they have mine and the whole Scottish Conservative Party.

"We should all be focused on what we can do to help the Ukrainian people in any capacity."

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Ian Blackford, the SNP's Westminster leader, branded Mr Ross's decision to withdraw his letter of no confidence as an "utterly humiliating U-turn".

"Only a few weeks ago he was categorical that Boris Johnson should be removed from Downing Street over his repeated rule-breaking," he said.

"Now apparently he will roll out the carpet for the prime minister at the Scottish Conservative conference and pretend that the no-confidence letter that he submitted with such fanfare never happened."

Mr Ross was not alone among Tory MPs to submit a letter of no confidence in the PM to Sir Graham Brady, the chair of the Conservative Party's powerful 1922 Committee, over the partygate row.

A total of 54 letters are needed to trigger a confidence vote in Mr Johnson's leadership, although Sir Graham does not comment on the number of letters submitted unless that mark is reached.