In a remarkable leaked letter, the Business Secretary Vince Cable has ripped apart the government's strategy on growth and other key economic policies.

The four page letter (which you can read in full here) was published yesterday by the BBC. Date 8th February and addressed to David Cameron and Nick Clegg, the private letter warns that "market forces are insufficient for creating the long term industrial capacities we need" and says there is "no connected approach across government". In a brutally frank assessment, Cable writes:

I sense however that there is still something important missing: a compelling vision of where the country is heading beyond sorting out the fiscal mess; and a clear and confident message abut how we will earn our living in future... We can be more strategic and the economic backdrop will increase demands that we are ambitious.

He makes several suggestions, the most radical of which is that the majority taxpayer owned Royal Bank of Scotland is broken up to create a "British Business Bank with a clean balance sheet and a mandate to expand lending rapidly to sound business".

But, predictably, it is not Cable's suggestions which have drawn the most attention, but his tough criticism of the way his own government -- and in places, his own department -- have handled the economy. This lays bare tensions in the coalition, at a particularly crucial time as the Budget draws near.

There was some irritation in the Treasury at the timing of the leak, as tense negotiations over the Budget continue. Downing Street and the Deputy Prime Minister's office both downplayed the importance of Cable's intervention, but a range of Conservative MPs - amongst whom the Business Secretary is already unpopular - expressed their annoyance.

Brian Binley, a Tory member of the Business Select Committee, summed up the feelings of many, telling the Times (£):

I am bitterly disappointed that the man put in charge of the growth agenda feels there is a problem with the agenda.

I find him a pleasant person to deal with. But if he is disappointed by the lack of growth, he is in as good a place as anybody in the Commons to make that clear. If he is struggling to do that, perhaps he should step aside and allow someone else to have a go."

Commentators across the political spectrum have been quick to notice that Cable's suggestions -- interventionism, patriotism, and supporting winners -- closely echo Labour's line. Yesterday, Ed Miliband gave a speech on "industrial activism". My colleague George Eaton noted the concordance between the views touted publically by Miliband and Cable yesterday.

This letter will strike a nerve with Tory top command precisely because it is close to the bone. The charge they are most sensitive to is that they have failed to articulate a strategy for growth, or indeed any economic strategy beyond deficit reduction. It is more damaging because it comes from within the government, even if it is from Cable, Liberal Democrat and eternal discontent. Yet, on the other hand, many Conservative commentators are raising the question: as Business Secretary, isn't Cable at least partly responsible for creating this situation? The shadow business secretary Chuka Umunna has been fast to say that the letter shows that "David Cameron and George Osborne have become roadblocks to the modernisation and reform needed to create a more productive economy."

The blame game will continue in Westminster, but outside it is largely irrelevant. What matters is that these criticisms sting because they are founded in fact, and the government does not have any decisive answers.