Boris Johnson’s main adviser on battling Scottish independence has departed less than a fortnight after being made the head of Downing Street’s Union unit.

Oliver Lewis, a veteran of the Vote Leave Brexit campaign, is reported to have said his position was made “untenable” by others within No 10.

Mr Lewis replaced former Scottish MP Luke Graham as head of the unit earlier in February, just months before Holyrood elections where calls for a second independence referendum will be the dominant issue.

Following Mr Lewis’s departure, Scottish First Minister Nicola Sturgeon tweeted: “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 claimed Mr Johnson’s “taxpayer-funded anti-independence campaign is completely falling apart”.

“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.