Fitch Ratings has warned it may cut the UK's AAA sovereign foreign currency credit rating.
It cited higher-than-expected government debt levels and downward revisions in its economic growth forecasts.
Fitch placed the rating on review for possible downgrade and said it saw a heightened possibility of a downgrade.
A decision on the rating level is due by the end of April, Fitch said in a statement.
The review comes hard on the heels of the government's annual budget, which halved Britain's growth forecast for this year and raised borrowing projections.