Oliver Lewis, who previously worked on the Brexit campaign, has resigned, according to The Times. The former head of the unit, Luke Graham, stepped down at the start of this month. Scotland’s First Minister and SNP leader Ms Sturgeon suggested Mr Lewis had quit after “realising how threadbare the case” was for the Union.

She wrote: “Disunity in the Union unit.

“Or maybe just despair at realising how threadbare the case for it is.”

The SNP’s Westminster deputy leader Kirsten Oswald also viciously taunted Mr Johnson and his Conservative Party.

She said: “As support for independence grows, the Tories are losing advisers like rats on a sinking ship.

“People in Scotland have a right to determine their own future in a post-pandemic referendum. Boris Johnson knows he cannot deny democracy any more than Donald Trump.

“The issue at the election in May will be this: who has the right to decide what sort of country we should be after the pandemic – the people of Scotland or Boris Johnson?

“The only way to ensure Scotland’s future is in Scotland’s hands not Boris Johnson’s is with both votes SNP.”

Downing Street declined to comment on staffing matters.

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