Disappointing revenues from North Sea oilfields and overshoots of government spending plans may mean that Alistair Darling will face an increased struggle to meet Treasury forecasts for lower borrowing this year, economists said yesterday.

As the Chancellor prepares to deliver his first PreBudget Report in October, official figures showed that in the first four months of the 2007-08 financial year, borrowing is already running ahead of last year’s levels.

Public finance figures for July, released yesterday, indicated that over the four months since April, the Treasury’s net borrowing stands at £10.1 billion – £800 million higher than over the same period in 2006-07.

At the same time, the surplus on the Government’s current budget, which measures day-to-day, noncapital spending against tax receipts, stood at £4.5 billion in the four months to July, £1.3 billion higher than in the same period last year.

The figures suggested that a combination of more spending restraint and stronger revenue growth is needed if Mr Darling is to meet the forecasts set out in the March Budget for borrowing over 2007-08 to drop modestly to £34 billion from £35 billion in 2006-07.

The shortfall so far this year was blamed on lower than anticipated receipts of corporation tax and petroleum revenue tax from North Sea oil, alongside somewhat faster than intended growth in public spending.

But the independent Institute of Fiscal Studies (IFS) said that the weakness of North Sea revenues was not expected to persist and joined other analysts in cautioning that, with two-thirds of the financial year still to come, it was too soon to draw strong conclusions for the full financial year.

The IFS also seized the opportunity ahead of the Chancellor’s October statements to call for Mr Darling to overhaul the much-criticised fiscal rules bequeathed to him by the Prime Minister. While the IFS said that, after controversial adjustments to the measurement of the economic cycle over which Gordon Brown’s so-called “golden rule” is gauged, he had met the letter of this stricture, as defined by the Treasury, it should be rethought.

Under the present golden rule regime, the Government must fund all noncapital spending from tax receipts over an economic cycle. But this has led to criticisms that the definition is too susceptible to manipulation to flatter historic figures, not least by altering the estimated dates of the cycle.

Carl Emmerson, deputy director of the IFS, called for Mr Darling to overhaul the rule to make it more forward-looking, less dependent on dating the cycle and to make explicit the uncertainties surrounding forecasts.

He also called for a broader measure of government liabilities and debts, to make clearer the future burden of public sector pension plans, and funding of projects under the private finance initiative.

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