A key Government pledge on debt policy appeared to be left in tatters today after at least £90 billion of Northern Rock's liabilities were officially brought on to the public sector's balance sheet.

The move, which covers all of the stricken Tyneside lender's wholesale borrowings, securitisations issues and customer deposits, makes it almost certain that the Treasury's "golden rule" on the ratio of debt to GDP will be breached.

It comes only weeks before Alistair Darling, the Chancellor, is due to present his annual Budget.

The Government's "sustainable investment rule" states that debt should not represent more than 40 per cent of GDP. The ratio stood at 37.7 per cent in December, with public sector net debt at £536.5 billion.

The Treasury declined to comment on whether the rule would be breached.

Martin Kellaway, a National Statistics statistician who co-authored a 26-page explanation of the reclassification, said Northern Rock's finances had been subject to scrutiny since November.

He said National Statistics' estimate that the Rock's liabilities were roughly £90.7 billion for the purposes of the Government's balance sheet were based on its report and accounts as at the end of 2006.

He said it was not clear whether the actual figure to include the period since then was higher or lower. It will take two to three months for National Statistics to establish a precise number for the liabilities.

Previous total taxpayer exposure to the Rock has been estimated at more than £57 billion, equivalent to a cost of about £2,000 for every Briton.

Coming only a week after the Institute for Fiscal Studies, a respected think tank, gave warning that a hole in Government finances meant that taxes would have to go up by £8 billion, it represents yet another blow to the economic credibility of Mr Darling and Gordon Brown, the prime minister.

The Treasury declined to comment on whether plans for the Budget would be effected by today's action.

A spokesperson for the Treasury said: "As the Chancellor has said, any impact would be temporary and exceptional and the code for fiscal stability has provisions for such situations.

"We will report on the fiscal position and our assessment of performance against the fiscal rules at the Budget in the usual way."

Sources close to the Treasury were relaxed about the reclassification, stressing that the rule was put in place to act as "intergenerational" protection and that any breach would be temporary.

The Office for National Statistics said the embattled Tyneside mortgage lender would be reclassified as a public financial corporation as from last October, in the wake of the Treasury's promise to underwrite its emergency borrowings from the Bank of England.

The Bank's financial position will also be included for statistical purposes, which would mean the Bank and Northern Rock's transactions would effectively cancel each other out.

Although National Statistics said the reclassification "should not be confused with 'nationalisation', which is a term commonly used to refer to public ownership", the move would smoothe the process for the Government should it decide to pursue that route.

Goldman Sachs, which has been advising the Treasury, has put together a plan that would see Rock debt, underwritten by Government, sold on the public markets.

Northern Rock has been in financial crisis since its emergency Bank loans emerged in September. It found itself in serious financial trouble after becoming over-reliant on borrowing on the wholesale markets.

Earlier this week Sir Richard Branson's Virgin Group and a Rock management team fronted by Paul Thompson submitted rescue proposals.

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