BORIS Johnson is at the centre of a row over a sustained series of attacks on the funding system used to calculate public spending in Scotland.

The former Foreign Secretary, who is the favourite to become the next Prime Minister, described a Conservative promise to keep the Barnett Formula as “reckless” and said it would be better spending money in Croydon than Strathclyde.

He also wrote that the same funding system amounted to a “present” from the English taxpayer and hit out at plans during the independence referendum to boost the Scottish Parliament’s financial powers.

Ian Blackford, the SNP Westminster leader, said: “These comments serve as a stark reminder of Boris Johnson’s utter contempt for Scotland. His string of ignorant remarks over the years are incredibly offensive to people across Scotland and other parts of the UK outside London – suggesting they fail to contribute to the economy and rely on only the generosity of London to get by, when the reality is that Scotland more than pays its own way."

Asked about his current view on the Barnett formula, a spokesman for Johnson did not respond.

Paul Hutcheon: The SNP will be the winner if Boris becomes PM

Johnson is the frontrunner to become the next Conservative leader and Prime Minister following the departure of Theresa May, who announced her resignation on Friday.

He had already declared his candidacy and will be pitted against MP colleagues including Dominic Raab, Sajid Javid, Jeremy Hunt, Esther McVey, Michael Gove, Rory Stewart and others.

MPs will whittle down the contenders to a shortlist of two, after which Conservative party members will be balloted to select a winner.

However, although Johnson is the darling of the Tory grassroots, he is less popular amongst his MP and MSP colleagues in Scotland. They worry that a victory for him may result in support for independence increasing.

This newspaper has pieced together Johnson's long-term hostility to the Barnett Formula, a system that gives Holyrood a population share of public spending increases in Whitehall. Ending it could usher in deep spending cuts.

The methodology, named after the late Labour peer Joel Barnett, has been in place since the late 1970s and results in higher spending in Scotland. The peer distanced himself from the system in the years before his death and MPs representing English constituencies have complained about its fairness.

One of the chief critics of Barnett has been Johnson, a former London mayor who now represents a borough constituency in the UK capital.

Herald on Sunday opinion: Boris Johnson must call a General Election

Days after the independence referendum in September 2014, Cameron, who was Prime Minister, insisted that Barnett would be protected. Johnson responded:

"I'm very keen on a Barnett formula which does justice to Barnet with one T [Barnet in north London]. We can't just go on with a system that even Joel Barnett himself thinks is outdated.

"I think there is quite a good way of honouring this slightly odd promise and that is to ask him to redesign it. I'd like to see him or someone else have a go. That's my solution for honouring what I see as a slightly reckless promise."

He also criticised the cross-party plan, made during the referendum, to equip Holyrood with greater powers: “Ever more things we are giving Scotland. Alex Salmond has been thrashed in these debates – but for no reason we are promising the Scots more tax-raising powers. There's no need to do it. What has England ever got out of this devolution process?”

Two years earlier, he said: "If London isn't moving, the UK is nowhere. A pound spent in Croydon or Tottenham, the areas hit by the riots last year, will generate far more for the rest of the economy than a pound spent in Strathclyde."

He also said: "In the end you'll create more jobs in Strathclyde or Derby by investing in London than you will by investing in Derby. This is the place that drives the rest of the UK. If it causes Celtic wailing, then I'm willing to go there and make the case that it's right for them too.”

Johnson also criticised Barnett in 2009, describing it as a “system of amazing political antiquity by which the English taxpayer sends about £20 billion every year to Scotland as a kind of present”.

He also took aim in 2006: "It is becoming clearer and clearer to people that they are being short-changed. The Barnett formula is producing deeply inequitable outcomes”.

READ MORE: Tories face the choice of living dangerously with Boris, or dying quietly with Jeremy Hunt

Labour MSP James Kelly said: “Any attempt by Boris Johnson to weaken the current Barnett arrangements will be viewed as an attack on public services in Scotland.

“Scotland does not need a right wing, Eton educated Tory PM who is detached from the real issues in Scotland’s communities. Scotland needs a General Election and the return of a Labour government which will invest in public services and tackle rising levels of inequality.”

Meanwhile, Stewart, who was recently appointed to Cabinet by May, launched an attack on Johnson’s Brexit strategy.

Johnson, a Brexiteer, last week suggested the UK could leave the EU without a deal, an outcome Stewart criticised.

Stewart told the BBC that a no-deal exit would be "damaging and dishonest".

He said: "I could not serve in a government whose policy was to push this country into a no-deal Brexit.

"I could not serve with Boris Johnson. I spoke to Boris, I suppose, about two weeks ago about this and I thought at the time he had assured me that he wouldn't push for a no-deal Brexit.

"So, we had a conversation about 20, 25 minutes and I left the room reassured by him that he wouldn't do this. But, it now seems that he is coming out for a no-deal Brexit."

Stewart said there was no majority in the Commons for no-deal, stating: "I think it would be a huge mistake. Damaging, unnecessary, and I think also dishonest."