Today another hammer is taken to the crumbling edifice that is the Barnett Formula.

Today another hammer is taken to the crumbling edifice that is the Barnett Formula.

The wielder this time is the ippr, a self-styled progressive think tank, which says the 30-year-old system of sharing out money across Scotland, Wales and Northern Ireland is unfair, inefficient, unaccountable and untenable.

Indeed, it warns that if Gordon Brown does not grasp the thistle, then his much-beloved Union could be under even greater threat because of the perceived sense of injustice, particularly from the English.

The formula is a political can of worms because, horror of horror for any government, reforming it entails creating winners and losers. Or as the report notes it "institutionalises infinite blame potential".

So, no-one in their right mind would seek to open the can when it could cost them dearly. Hence, the extreme caution with which ministers view the subject. They currently insist that the formula has served the country well and, so, why change it?

As the ippr report points out: "The Barnett Formula is not based on needs but on historic spending patterns and population."

This means when there is extra departmental spending in England, then Scotland, Wales and Northern Ireland gets a proportionate slice of the action.

But the report also points out that, on occasion, there is an arbitrariness about when Treasury spending is or is not subject to the Barnett Formula rules.

It notes, for example, how the SNP has got all hot and bothered over the fact that spending on the 2012 Olympics has not been "Barnetted", nor indeed has £1.2bn spending on prisons in England.

The ippr's solution is not full fiscal autonomy, which, it points out is tantamount to independence, but rather a hybrid solution of allowing Holyrood to raise income tax and get a share of other taxes such as VAT plus a top-up grant from Whitehall based on Scotland's needs.

Of course, the Calman Commission is looking at all of this and is likely to come up with some sort of extra powers solution; after all, the opinion polls show most Scots like the idea of having more power while, at the moment, staying as part of the Union.

Wendy Alexander, the now ex-leader of Labour in Edinburgh, is on board for change, as are many English Labour MPs who feel Scotland gets too large a slice of the UK pie. David Cameron, the Tory leader, has also signalled that the game is up for Barnett. Yet the UK Government remains devotedly tied to it.

Officially, it says the formula has worked well, has served Scotland's, Wales's and Northern Ireland's interests for a generation and so, if it ain't broke, why fix it?

However, one senses the sands are shifting albeit slowly.

The PM recently said - to raised eyebrows: "There is an issue about financial responsibility of an executive ... that has £30bn to spend but doesn't have responsibility for raising(that).

"In any other devolved administration in the world, there is usually a financial responsibility that requires not only the spending of money by the administra-tion but also its responsi- bility to take seriously how it raises it."

Post-Calman proposals to change the devolutionary settlement will have a knock-on effect on Barnett. The expectation is the commission will propose giving Holyrood more power over tax-raising to increase accountability - just like the ippr today recommends.

The Tories are likely to accept this and put in their Westminster manifesto a desire to scrap the formula, hold a UK wide needs assessment with a view to establishing a new system. The question is will Mr Brown do likewise?

It will be a tough call because it could mean Scotland getting a thinner slice of the cake with all the political ramifications that could mean. The ensuing debate will throw up all manner of political demons and, as ever, at its heart will be the survival of the Union.