According to a report in 'The Times' newspaper, despite the £1.25 billion in backing for the CTRL having been raised by London and Continental Railways (LCR) through a bond issue to private investors, the ONS ruled that it must count towards the national debt.

'The Times' reported that the bonds issued by LCR are financed through a securitisation of future income to be paid to the company, in part by the Government.

In a conclusion that highlighted controversy over the Treasury's use of so-called "off-balance sheet" finance, the ONS said: "The mechanics of the set-up are such that government is judged to be holding the risk rather than the bondholders."

So far, over £20bn of borrowings by Network Rail, guaranteed by the government, have not been counted by the ONS as counting against government expenditure.

But with the ONS decision on the CTRL borrowings highlighting the issue again, there was City speculation that Chancellor Gordon Brown may have to alter his rules.