Fitch announced on Friday that it has placed Britain's top-level AAA credit rating on watch for possible downgrade because the government's latest forecasts expect debt to peak later and at a higher level than previously expected.
"Fitch Ratings has placed the United Kingdom's 'AAA' Long-term Issuer Default Ratings on rating watch negative (RWN) indicating a heightened probability of a downgrade in the near term," it said in a statement.
The ratings agency said it plans to complete its review of Britain by the end of April.
The news came one month after rival agency Moody's stripped Britain of a triple-A debt rating, saying government debt was still mounting and that growth was too weak to reverse the trend before 2016.
Fitch said "the RWN reflects the latest economic and fiscal forecasts published by the Office for Budget Responsibility that indicate that UK government debt will peak later and at a higher level than previously expected".
Gross government debt is now forecast to peak in 2016-17 at 100.8 percent of gross domestic product before starting to decline.
"Fitch has previously stated that GGGD failing to stabilise below 100 percent of GDP and on a firm downward path towards 90 percent over the medium term would likely result in a downgrade of the UK's sovereign ratings," it added on Friday.
"Fitch's decision to put the UK's AAA rating on negative watch outlook is entirely predictable," said Howard Archer at IHS Global Insight
"And there can be little doubt that the agency will follow through with a downgrade to AA," added Archer, chief UK and European economist at the economic and financial analysis firm.
Earlier this week, on Wednesday, British finance minister George Osborne unveiled another painful austerity budget that also slashed economic growth forecasts and raised borrowing estimates.
The economy is now expected to grow by just 0.6 percent 2013, compared with previous guidance of a 1.2-percent expansion, according to the OBR.