Britain's prized AAA credit rating remains under "significant pressure", the Fitch rating agency has warned.
David Riley, the head of global sovereign ratings at the agency, said Chancellor George Osborne's admission in last month's autumn statement that he would miss his 2016 target date for public debt to start falling had been a "negative event" for the UK.
While he stressed that a downgrade of the AAA rating was not a "decided event", Mr Riley said Britain remained "vulnerable" to fresh economic "shocks" elsewhere in the world.
"There is increasing concern that the fiscal consolidation is happening more slowly, that the economy isn't recovering as quickly as we had hoped," he told BBC Radio 4's The World at One.
"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."
While he said that there were still some strong underlying economic "fundamentals" to support the AAA rating, 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."
Any downgrade to the UK's rating would be a huge blow to Mr Osborne who has placed great emphasis on the importance of maintaining the AAA status.