A furious dispute inside Downing Street over the UK government’s strategy to counter growing calls for Scottish independence has resulted in the departure of Boris Johnson’s chief adviser on the union.

Government officials confirmed Luke Graham, the former MP for Ochil and South Perthshire from 2017 to 2019, was quitting the post and that Oliver Lewis, a key Tory figure in last year’s Brexit trade negotiations, would be leading what one of the officials said would be a “beefed-up union unit”.

The Number 10 row which resulted in Mr Graham’s exit erupted last Friday — a day after Mr Johnson visited Scotland to bolster support for the three centuries old union.

“It was fairly brutal,” admitted one government insider.

Mr Lewis, known in Downing Street as “Sonic”, is said by colleagues to have wanted “a clean slate” and to build a new team. A Vote Leave veteran, he is seen as having strong campaigning credentials although he is not known for any particular expertise in Scottish politics. He threatened to resign over the departure of Mr Johnson’s former senior adviser Dominic Cummings in November, but was persuaded by the prime minister to stay in Number 10.

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

The future of the UK has become a central challenge for Mr Johnson, after opinion polls have suggested sustained support for Scottish independence. Mr Lewis’s role will be central to Number 10’s efforts to counter the Scottish National Party.

According to officials with knowledge of the dispute, Mr Graham was unhappy with both his role and Number 10’s strategy for the union. “There was a lack of faith in letting him do what he needed to do. A lot of his efforts were frustrated by other people in the building,” one said.

One Scottish Tory said: “Luke was the only one who gets Scotland in there, it’s a big shame he’s gone and speaks to their total lack of a strategy on how to deal with the SNP.”

Mr Graham will now move to Scotland to assist the Scottish Tory party’s campaign for May’s Holyrood elections.

Another Whitehall official said: “Luke Graham has improved the focus of the government on delivering for Scotland and all parts of the UK and has helped ensure communications focused on demonstrating the practical value of the union to people in all parts of the country.”

But others in Number 10 put the blame on Mr Graham for his exit. “Luke had gradually pissed off more and more people. He didn’t endear himself to other spads [special advisers], ministers or civil servants. No one was weeping to see him go,” one government official said.

Downing Street declined to comment on “personnel matters”. Mr Graham also declined to comment on his departure. One individual with knowledge of the row said: “They did such a bad job of sacking him, he didn’t realise he had been sacked. The HR was appalling.”

Mr Graham’s departure comes as Number 10 seeks to tackle the rise in support for secession. A review by Andrew Dunlop, former Scotland Office minister, into how to make the union function more effectively has still not been published despite being completed in November 2019.

Meanwhile, the UK government has offered assistance to the Scottish government with the rollout of Covid-19 jabs, after its vaccination rate continued to fall behind England. About 19 per cent of adults in England have received their first dose, compared to 14 per cent in Scotland.

Alister Jack, Scotland secretary, wrote to Scotland’s first minister Nicola Sturgeon to offer his “full support in rolling out the vaccination programme and fighting the pandemic across the whole country”.

Mr Jack added: “With that in mind, we stand ready to offer any support or assistance we can give you to accelerate your rollout programme.”