We all know that one swallow does not make a summer. But what about a swallow, a cuckoo, a lark and a dove? As Alistair Darling puts the finishing touches on what is expected to be the grimmest Budget since the dark days of Norman Lamont and John Major, let us hope he attends to the green shoots of economic recovery, rather than worrying about the debt burdens that he may or may not impose on generations yet unborn.

There is only one question that the Chancellor really needs to consider on Wednesday - and it is not whether the Treasury should spend £2billion on subsidies to car buyers. It is whether the economy can return to adequate growth within the next 12 months.

If it does, then all the prophecies of doom about the Treasury's impending bankruptcy will soon be forgotten, along with the predictions of enormous tax increases and unparalleled cuts in public spending in the coming decade. If, on the other hand, the economy remains paralysed for much longer or experiences only a feeble recovery when growth resumes, then it won't matter what tax or spending measures Mr Darling announces: public borrowing will grow like Topsy and, in the end, the Government's only recourse will be to inflate away its debts by ordering the Bank of England to print money without limit.

To see what I mean, consider the pre-Budget analysis published by the Institute for Fiscal Studies last week. The shock-horror part of this analysis was its estimate that taxes might have to rise by £39billion annually, or £1,250 for every family in Britain, on top of the £38billion of tax increases already announced in the November Pre-Budget Report. The purpose of this big additional tax increase would be to achieve a budget balance by 2015-16, an arbitrary date that the Treasury and the IFS plucked out of the air.

So far so horrible, but the IFS analysis also noted that the enormous deterioration of Britain's deficit in the past year has had almost nothing to do with Treasury decisions on tax and spending.

The stimulus package announced last year has accounted for just one fifteenth of the increase in this year's budget deficit according to the IFS. The rest was caused by the collapse of growth and the shift in economic activity from financial services to other parts of the economy that are far less heavily taxed. Despite all the headlines about tax havens and bank-accounting scams, it seems that the financial services provide far more than their fair share of Britain's total tax take. This is why public finances have deteriorated much more rapidly in Britain than in other advanced economies, even though the British economy as a whole has suffered rather less from this recession than Germany, Japan or the US.

But there is another side to the horrendous impact of recession on Britain's public finances. If the economy recovers more quickly than expected the deficits will decrease of their own accord.

Bear in mind that official forecasters are no better at predicting recoveries than recessions: every recession in the past has been followed by one or two years of rapid growth, well above the economy's long-term 2.5 per cent growth trend. This dramatically changes the fiscal picture, but it does not appear in the the Treasury or IFS projections.

And, given the signs of stabilisation that are starting to appear in the housing market, the consumer-spending figures and the figures on bank lending, a quicker than expected recovery is becoming a reasonable expectation.

The natural narrowing of deficits will be even faster if finance and housing prove more resilient than the Treasury and IFS assume. If they do prove more resilient, then tax revenues will rise sharply. Also if, as many commentators now hope and expect, the structure of the British economy does shift permanently in favour of exports and manufacturing, then it would surely be reasonable to expect a change in the tax structure, designed to extract more revenues from these newly buoyant sectors, instead of struggling homeowners and banks.

It is equally possible, on the other hand, that any revival of the world economy will be accompanied by a resumption of growth in international finance, a rebound in asset markets and a gradual recovery in house prices not to their overinflated levels of the boom years but to somewhere well above the depths now assumed as permanent in many projections. If this happens, then government revenues will also recover.

The idea that public finances could improve of their own accord alongside an economic recovery, without any need for the Chancellor to announce tax increases or spending cuts in next week's Budget, may sound like irresponsible wishful thinking. Can I really be recommending that the Treasury should just carry on borrowing and hope for the best? The answer is yes. Hoping for the best is a more responsible and realistic policy than committing many years in advance to large tax rises or public spending reductions. There are three reasons for this apparent paradox.

First, the announcement of draconian tightening in future tax and public spending policy can depress business and consumer confidence, damaging the prospects for recovery. This is not always the case. In a normally growing economy, announcing budgetary plans in advance can help to maintain financial stability. If fears of inflation are rampant, as they were in the 1980s and early 1990s, reducing budget deficits can also bring down interest rates, boosting growth. But when the economy is in deep recession, ensuring a recovery has a much bigger impact on future budget deficits than any conceivable change in tax or spending plans.

The second reason is to do with the nature of long-term economic and fiscal forecasts. Quite simply, they are always wrong and in the depth of recession or the height of a boom, the forecasting errors are enormous. Because nobody has the faintest idea how big deficits will be in 2015, it is dishonest to pretend that any actions today can control these illusory numbers.

Which brings us to the third and most persuasive reason why any tax increases or spending cuts announced next week would be pointless: no government can bind its successors. Fiscal projections for 2015 relate to the Parliament after the next one. This means that the political credibility of any long-term announcements next week would be zero.

The best thing that the Chancellor can do is also the best he can do for the economy. He can point to the green shoots of recovery - and make sure that he does them no harm.

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