By Nyta Mann BBC News Online political correspondent

But what is beyond doubt is that it has made its way off the backburner and back onto the front pages.

The relationship between the prime minister and the chancellor has been and remains one of the great strengths of this government, whatever people may claim

Tony Blair's spokesman

So much so that on Monday morning Downing Street was moved to declare the relationship between the prime minister and his chancellor one of the "great strengths" of the New Labour government.

No one would argue with that. The problem for Tony Blair is that the Blair-Brown relationship is also perilously close to being one of its most destabilising factors.

The latest spark to light the flames is the euro. The government's famous five tests on whether to join the single currency are due to be concluded in June.

Rift extends beyond the euro

All the signs from Gordon Brown are that his assessment will be a negative one and a euro referendum should not take place in this parliament.

Gordon Brown: Policy differences with Blair

Mr Blair, meanwhile, has consistently struck a less euro-sceptic note. Downing Street also sees his chance to leave his mark on history with the issue receding with any further delay.

But of course, it is all about much, much more than just the euro. It always is.

The prime ministerial succession, and whether Mr Brown will get it, is an underlying seam that has run through New Labour since before it took office in 1997.

As well as the euro, top-up fees, foundation hospitals and possible military action against Iraq are just the other policy flashpoints that have raised the temperature of late.

Mr Blair backs them all; Mr Brown has reservations.

But what is different about the latest outbreak of tension between the two allies-cum-rivals is the manner in which Mr Brown's differences have become apparent.

Differences blatantly expressed

The standard proxy briefings have taken place, with trusted camp followers of both lauding their man while sticking the knife in the other.

It is a lot more blatant this time round, however.

And only last month Ed Balls, Mr Brown's close Treasury aide and all-round political familiar, gave an extremely rare newspaper interview in which he said that applying market principles to health and education would "run grave risks with the ethic of public service".

At the height of an internal government row over the creation of two-tier health and education services, this was a clear shot from Mr Brown not so much across the prime minister's bows as straight at them.

There is renewed speculation as to who gets to inherit the crown from Tony Blair

Most remarkable, however, are Mr Brown's comments at a breakfast meeting of executives from the Guardian newspaper.

Unless the reports are untrue - and note that Mr Brown has not denied them - the chancellor chose this meeting with journalists to baldly express his opinion that top-up fees are a "ridiculous idea".

Without making assumptions as to the chancellor's motives, this is not the behaviour of a politician desperate to avoid speculation reaching the press about disagreement with the prime minister.

Clock ticking for Brown

So what is he up to, taking the fight towards the open ground when it has previously been conducted in the shadows of government?

The policy differences being expressed now are not manufactured and few believe they provide sufficient explanation.

But the clock of history is ticking and Mr Brown has already held the title of Next Prime Minister for more than five years.

It is in many ways a dangerous position to be in - certainly for any length of time and particularly when your waiting-room is the Treasury.

The longer Mr Brown is delayed, as his supporters see it, from taking his rightful role, the less likely he is to get it. Events take their toll, the economy turns against you and the opposition gradually gets its act together.

Mr Brown is widely acclaimed as the cleverest man in the cabinet and its ablest politician.

He knows his history too, and will have noted that the longer Mr Blair goes on, the more likely the crown is to slip from the grasp of the man set to inherit it.

That would be enough to try the patience of any man, let alone an ambitious politician.