JEREMY Hunt has ramped up the pressure on Boris Johnson in the race to become the next Prime Minister after backing the controversial system that gives Scotland a public spending advantage over England.

The Tory leadership contender told The Herald on Sunday that it is “essential” to maintain the Barnett Formula, which Johnson has criticised for over fifteen years and claimed is unfair on places such as London.

One of Hunt’s closest supporters also urged Johnson, who will take part in a hustings in Scotland this week, to back the call.

In another development, a spokesman for the Scottish Conservatives said: “The Barnett Formula works for Scotland and works for the UK. Any candidate must either commit to it, or set out a very detailed plan for an upgrade.”

The Barnett Formula, named after the late Labour peer Joel Barnett, was introduced in the 1970s and allocates public spending increases to the Scottish Parliament.

Based on population size, rather than need, the system has resulted in Scots receiving a far higher level of spending per head in areas such as health and education.

Supporters believe scrapping it could lead to swingeing cuts north of the border and undermine a key plank of the argument for Scotland staying in the UK.

Holyrood’s funding system has now become a dividing line in the battle between Hunt and Johnson to become the next Conservative leader and Prime Minister.

Johnson, a former mayor of London, has a track record of hostility to the Barnett formula which goes back to 2006 when he said: "It is becoming clearer and clearer to people that they are being short-changed. The Barnett formula is producing deeply inequitable outcomes”.

Three years later, Johnson stepped up his attacks by describing Barnett 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”.

In 2012, he called for funding to be diverted from Scotland to London:

"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.”

His most recent criticism came in the days after the 2014 independence referendum when the then Prime Minister David Cameron said the system 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."

Hunt, who has stressed his Unionist credentials in the contest, has intervened to say he will keep the system as Prime Minister.

His spokesperson said: “The Barnett formula is essential to protect Scotland’s public services. That’s why a government run by Jeremy would maintain it, to help ensure Scotland’s schools and hospitals get the funding they need.”

John Lamont MP, one of Jeremy Hunt's original backers in his bid to become Prime Minister, went further: “There is no case for scrapping the Barnett formula. Boris said it was reckless of David Cameron to commit to keeping the Barnett formula. The only thing reckless would be scrapping it.

“This is yet another reason that Jeremy is the Prime Minister that Nicola Sturgeon doesn’t want. Boris should u-turn immediately and follow Jeremy in pledging to keep the Barnett formula. That is what’s best for Scotland and the Union.”

Senior Scottish Tories are worried that Johnson could end up backing a no-deal Brexit as Prime Minister, an outcome they fear would provide an electoral boost to the SNP. A majority of Tory MSPs, including Scottish leader Ruth Davidson, are backing Hunt.

In a challenge to Johnson, Davidson told the BBC last week: “I want to see him make assurances that it’s not Brexit do or die, it’s the Union do or die. That’s exactly what we’ve seen from the other candidate in the race and that’s why he’s going to get my vote.”

Johnson’s press team did not respond to an email.