Tuesday was Gordon Brown's big chance to open the floodgates and pour money into public services. He will have wanted to make the most of it, because economists say he may never have the opportunity again.

It is not that most analysts see the plans in the 2000 spending review as insanely reckless.

However, it probably represents a once-in-a-lifetime opportunity to deliver a rapid acceleration in the growth of public spending.

The big headline figures for aggregate spending and borrowing were all set out in the Budget in March, and Mr Brown has been true to his word that he would stay within that envelope.

Despite the International Monetary Fund's opinion that the Budget was "regrettably pro-cyclical", even Mr Mervyn King, the Bank of England's hawkish deputy governor, thought its total effect on economic growth and inflation would be modest.

The Bank of England's monetary policy committee was briefed on the key points of the review by Mr Gus O'Donnell, head of macro-economics at the Treasury, for its last meeting a fortnight ago.

Nothing the committee heard alarmed it enough to make it want to raise interest rates immediately.

One figure that might make the committee think twice is the growth in total government spending this year from last year: it will rise by 6.7 per cent in real terms, the biggest one-year increase since 1974.

Ms Kate Barker, chief economist of the Confederation of British Industry, said: "I wonder whether when the Bank comes to look at this again, it will raise the question of how rapidly the economy can grow in the short term."

But the Treasury says the sharp increase - a result of the undershoot of government spending last year - does not change the overall fiscal stance.

The public finances are forecast to swing from a surplus of £6 billion in the current year to borrowing of £13 billion in 200405, but that deficit is still only 1.1 per cent of gross domestic product.

It could all go wrong. Moderate variations in growth or even a recession could be accommodated within the Treasury's public finance framework by allowing borrowing to rise. However, a severe or prolonged downturn that forced a fundamental reassessment of economic prospects would create huge pressure for taxes to be raised or spending to be cut.

Undershooting tax revenues or overshooting benefit claims could also mean the Treasury's arithmetic would seem a lot less secure.

In the past few years tax revenues have been higher than expected and social security payments lower. In the early 1990s it was the other way around.

The Treasury's assumption of just 1.5 per cent annual real terms growth in social security spending may well turn out to have been over-optimistic.

There does not seem to be an enormous margin for error - an impression compounded by the decision to take all of the expected savings on debt interest and from lower unemployment, and add them to departmental spending.

"There is an air of getting as much as possible into this public spending envelope," Ms Barker said. "What worries us about these plans is that they rule out the possibility of tax cuts - they make them impossible for several years."

With those reservations aside, economists have little doubt the higher spending will be delivered.

"If there is no macro-economic disaster, then it all looks perfectly affordable," said Mr Martin Weale of the National Institute of Economic and Social Research.

The problem will come with the next spending review, in 2002. To have public spending growing at 3.3 per cent a year, while the trend growth rate of the economy is still around 2.5 per cent, is a position that cannot be sustained forever.

The proportion of national income spent by government in Britain is low by European standards, and even by 2003-04 it will still be lower than it was just before Labour came to power.

It is possible Labour will want to continue with its strategy of raising taxes to raise spending.

But by 2004 the budget deficit will begin to nudge towards the Treasury's upper limit, and there is no guarantee the economic outlook in the future will be as benign as it is now.

It is highly unlikely we will see another bonanza for the public services like the one unveiled.