A battle of wills within Downing St over the UK Government’s strategy to stem the tide of support for Scottish independence has led to an apparent outbreak of disunity with the departure of Boris Johnson’s chief adviser on the Union.

Luke Graham, the former Conservative MP for Ochil and South Perthshire, is said to have departed No 10 just a day after the Prime Minister’s controversial visit to Scotland, which Nicola Sturgeon had deemed “non essential”. One Government insider described the 35-year-old’s departure as “fairly brutal”.

It was only on Tuesday that Allegra Stratton, Mr Johnson’s Press Secretary, suggested Mr Graham was still the head of the No 10 Union Unit and was a “very, very, very valued member of staff”.

But Whitehall officials said he was being replaced by Oliver Lewis, nicknamed “Sonic,” who was a key Conservative figure in the Brexit trade talks.

As part of a planned “big push” on the Union ahead of the Holyrood poll in May, Mr Lewis is to head up what was described as a “beefed-up Union Unit”.

According to the FT, the former Vote Leave campaigner is said by colleagues to have wanted “a clean slate” and to build a new team.

Last November, Mr Lewis had threatened to resign over the departure of fellow Brexiteer Dominic Cummings, Mr Johnson’s controversial former chief adviser, but was persuaded to stay in No 10 by the PM himself.

“Oliver is good at this stuff,” declared one colleague. “He knows this can’t just be about brute economics. It’s about passion, identity and about promoting the idea that you can be proud to be Scottish and British.”

However, it is not clear what direct experience Mr Lewis has of Scottish politics.

With 20 consecutive polls placing the Yes campaign ahead, there is a degree of nervousness in Whitehall about the SNP’s momentum.

Last week, Alister Jack, the Scottish Secretary, insisted in an interview with The Herald that the Unionists were not losing the argument on the Union and that a pro-independence parliamentary majority in the Scottish elections was not guaranteed.

Mr Johnson, regarded by some Conservatives as an electoral liability in Scotland, has insisted he intends to make regular visits north of the border ahead of the May poll.

A supporter of Mr Graham said he was sorry to see him go as he was one of only a few people at the heart of the Downing St machine to have first-hand knowledge of Scottish politics but that he had become increasingly frustrated at the Government’s strategy for saving the Union.

However, a detractor noted how the ex-MP had “gradually pissed off more and more people” and that “no one was weeping to see him go”.

Earlier this week, No 10 sought to knock back rumours Mr Graham had left the Union Unit. Asked about his supposed departure, Ms Stratton told a daily media briefing: “We don’t comment on personnel issues inside No 10.”

Pressed on whether the former Scottish MP had been replaced, she replied: “That is not my understanding. Luke Graham is the head of the No 10 Union Unit…He is a very, very, very valued member of staff.”