The rating agency Fitch has warned that it may downgrade the UK's credit rating in April.
It has put the UK's long-term rating on Credit Watch Negative, which it said showed "a heightened probability of a downgrade".
The agency said it was due to higher-than-expected debt levels and downward revisions to UK growth forecasts.
The review follows the Budget this week in which UK growth forecasts for 2013 were halved.
The Office for Budget Responsibility cut its growth forecast to 0.6% for 2013, from 1.2% in December.
The pound fell sharply against the dollar after the news from Fitch, dropping half a cent against the dollar. It has since come back to trade at around $1.5222.