Fitch, the credit ratings agency, has warned the chancellor that Britain could be stripped of its prized AAA status if he fails to boost the country's economic situation in the spring budget
The agency said the UK remains under "significant pressure" following the autumn statement in December, when George Osborne conceded that growth would be lower over the next two years and for that reason he was likely to miss one of his two debt reduction targets.
David Riley, the head of global sovereign ratings at the agency, said Osborne's admission was a "negative event" for the UK.
While he stressed that a downgrade was not a "decided event", he said Britain remained "vulnerable" to fresh economic "shocks" elsewhere in the world.
Speaking on BBC Radio 4, he said: "There is increasing concern that the fiscal consolidation is happening more slowly, that the economy isn't recovering as quickly as we had hoped.
"This does leave the UK quite vulnerable either to a worsening of the situation in Europe or some kind of shock coming from the United States or elsewhere.
"It does mean that its AAA rating is under quite significant pressure."
Several economists have forecast that the UK will lose its status as one of a handful of countries that retain the much-coveted AAA credit rating. Investors have considered the UK a safe haven, which has allowed it to enjoy low borrowing rates. The fear in the Treasury is that the loss of the AAA rating would push up borrowing costs and make the UK's debt payments more expensive.
Fitch has also put the US and France on negative watch, though both countries have been downgraded by other ratings agencies without any discernable effect on their borrowing costs.
Riley said the UK could still boast some strong underlying economic "fundamentals" to support the AAA rating, but the situation could change if the budget in March showed a further weakening of the public finances.
"If we were, following the budget, to see that the debt is going up even higher than we currently project and is going to peak even later than we currently forecast, then that would put a lot of pressure in terms of the UK AAA rating because essentially, on a very broad measure, it would mean that the UK government is projecting a debt level of 100% of GDP," he said.
"In our view that is not really consistent with the UK retaining the AAA rating."
Riley warned the Obama administration it faces a "material risk" of losing its AAA status if there is a repeat of the wrangling seen in 2011 over raising the country's self-imposed debt ceiling.
The US hit its $16.4 trillion (£10.2tn) debt ceiling at the end of last year and is now employing special measures to meet its financial obligations. The treasury department said those steps could be exhausted by mid-February.
Despite December's deal in Congress to avoid the "fiscal cliff" of spending cuts and tax hikes, Riley said pressure on the country's rating was increasing. Standard & Poor's has already stripped the US of its AAA status, while Moody's and Fitch have put it on negative watch.
Riley said: "If we have a repeat of the August 2011 debt ceiling crisis we will place the US rating under review. There will be a material risk of the US rating coming down."
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