David Cameron today drops his biggest hint that a future Conservative government would scrap the 30-year-old Barnett Formula.

In an exclusive interview with The Herald ahead of today's Scottish Conservative conference in Ayr, the party leader makes clear the spending formula's days would be numbered under his leadership but is cautious about playing up to a "sense of English grievance" or doing anything to damage the United Kingdom.

The Tory leader was speaking of his aims as his party looked poised to continue its victory roll with a dramatic win in the Crewe and Nantwich by-election.

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Asked if it was time to get rid of the formula, Mr Cameron says: "This cannot last forever, the time is approaching ... If we replace the Barnett Formula with a needs-based formula, Scotland has very great needs and Scotland will get very great resources."

Asked if, therefore, the formula is coming to the end of the road, he replies: "Yes, that's right. I want this to happen in a consensual, sensible, non-inflammatory way and that's why I've been so reticent about it."

Thus far, the UK Government and Conservative Opposition have treaded carefully around the formula - which governs the distribution of additional public expenditure to Scotland, Wales and Northern Ireland - as it is a highly-volatile political subject. However, cross-party feeling is growing at Westminster and beyond that the funding scheme has to be replaced with a modern, fairer version.

The Tories have already called for a UK-wide needs-based assessment but the Treasury, which insists it has no plans to change the formula, will sometime this year publish an explanatory note setting out its mechanics - some regard this as paving the way for its eventual replacement with a system based more on need.

In the interview, Mr Cameron says Labour "completely screwed up on the Union with Bendy Wendy all over the place", referring to the row over the Scottish Labour leader's call for an early independence referendum. He suggests this has given the Scottish Conservatives an opportunity to be the beneficiaries of the Unionist vote in Scotland.

But the Tory leader notes: "If we succeed, we will help the Union succeed, if we fail - and in the past in Scotland we have failed - we let down the Union.

"There is a real link between making sure we are offering people a modern, successful centre-right party, putting forward progressive ideas about the future of our country, and the strength of the United Kingdom."

While Mr Cameron acknowledges Alex Salmond's "brilliance as a politician", he warns the First Minister against thinking that a Tory government in London would be a godsend to the SNP's goal of Scottish independence.

"If Alex Salmond thinks there's some clever game he can play about building on Scottish resentment against a Conservative government in England to help break up the Union, forget it. I will do everything I can to stop that from happening." He insists that because most Scots want to preserve the Union, Mr Salmond is ultimately on a "losing ticket".

Mr Cameron says he will not give in to those who argue that the Conservatives should jettison Scotland. "I don't want to be the Prime Minister of England, I want to be the Prime Minister of the United Kingdom. There's a very strong sense in the Conservative party that the United Kingdom is bigger than all of us."

As the votes were being counted in Crewe and Nantwich by-election last night, there were suggestions that the Tories will seek to capitalise on their expected win with a poll as early as next month in Henley, the Oxfordshire seat of MP Boris Johnson, the new Conservative London Mayor.

They will hope to build on the momentum set by his victory and the party's gains in the English and Welsh local elections.