Opposition parties at Westminster have argued that the announcement was an effort to woo Tory backbench MPs as they weigh up where to submit letters to the 1922 Committee demanding a vote of no confidence in Mr Johnson.

But Sue Webber, the Scottish Tories’ shadow public health minister, attacked Sir Gregor’s intervention, saying it was “inappropriate for a civil servant to be promoting political messaging”.

She added: “He has done good work throughout the pandemic, but Prof Smith should be above these SNP political attacks. We need to hear a clear explanation from the chief medical officer. I certainly do not understand how he could possibly think this is acceptable.”

Adam Tomkins, a former Tory MSP and a law professor at Glasgow University, tweeted: “This is the sort of partisanship you’d expect from Humza, but why is Scotland’s CMO, a supposedly non-political official, retweeting it?”

After deleting his retweet, Sir Gregor said: “Yesterday, I RT a message from the Cab Sec (Yousaf) as an attempt to contribute to the debate on the isolation period. However, on reflection it is clear this contained political messaging alongside the public health info. I have therefore withdrawn the RT and apologise for passing it on.”

A Scottish government spokesman said: “The chief medical officer has deleted the retweet and apologised. We have received the letter from Ms Webber and will respond in due course.”

Sir Gregor was knighted in the New Year’s Honours list alongside Prof Chris Whitty, his English counterpart.