The classification of £91 billion of Northern Rock liabilities as official government debt is largely symbolic. We already knew that taxpayers were on the hook if anything were to go wrong with the Rock rescue. The deliberations of the Office for National Statistics merely confirm the reality and show that the 40 per cent rule - Gordon Brown's and now Alistair Darling's badge of fiscal restraint - has been breached.

The fact that every household in Britain is liable for an extra £4,000 or so is resonant stuff, and something the Opposition has been quick to seize upon.

Such symbolic moments can make or break the reputations of Chancellors. No one can forget the pathos of Norman Lamont on the steps of the Treasury as he tried to put a brave face on sterling's humiliating expulsion from the exchange-rate mechanism. Denis Healey will for ever be remembered for his abrupt about-turn at Heathrow, when he headed back to the office to draft a begging letter to the IMF.

At City lunch tables, Darling, after just eight months in the job, is already being installed in the pantheon of accident-prone Chancellors. He has been dealt a lousy set of cards but he has also played them poorly. The absence of leadership in handling the Rock run and its aftermath and the botched decision on capital gains tax and subsequent dithering have lost him friends in British boardrooms.

The loss of personal information on 25 million child benefit claimants also came on his watch - again hardly his fault but an episode that resonated with the public, who have had to listen to too many sermons from authority on the importance of keeping personal data safe.

Things will come to a head next month. Mr Darling has to deliver his first Budget amid a steep deterioration in the public finances, and he has to put to bed, one way or another, the fractious child that is Northern Rock. Deft handling of both would immeasurably improve his standing; any further stumbles are likely to be treated harshly.

The public finances are already looking sickly despite the fact that the economy has until very recently been cantering along. The Institute for Fiscal Studies reckons even now Mr Darling will need to raise an extra £8billion in tax. A serious slowdown could only make matters much worse, depressing corporation tax, VAT and stamp duty receipts while lifting welfare costs.

He will struggle to both satisfy the critics demanding fiscal rectitude while responding to other demands that he act to soften the impact of a slowing economy. It is hard to see a positive Budget verdict on March 12.

A satisfactory outcome for Rock is an equal challenge. Keeping the shareholders sweet is going to be near impossible even if a rescue can be engineered. The irony is that the very reason the ONS has put the Rock's liabilities on the public books is that Mr Darling is deemed to have his hands firmly on the levers, yet Rock still has independent and obstreperous outside shareholders.

Structuring the bonds-for-loans financing without triggering a veto from Brussels, which has set a March17 deadline, is also going to be tough. Rival banks are already gearing up for serious protests if, as they fear, the cossetting of Rock gives it an unfair advantage.

March is going to be an inferno for the Chancellor. He will come through it with his reputation either tempered or toasted.

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