Ratings agency Fitch has downgraded its rating of Britain from AAA to AA+ with a stable outlook.
The agency said it had made the move due to "a weaker economic and fiscal outlook".
Fitch forecasts that the country's general government gross debt with peak at 101% of GDP in 2015-16, gradually declining from 2017-18.
It had previously predicted a peak of 97%, with that declining from 2016 onwards.
Last month Fitch warned that it could downgrade Britain's rating due to higher-than-expected government debt levels and downward revisions in its economic growth forecasts.
In February, Moody's Investors Service downgraded the country's bond rating one notch to AA1, saying sluggish growth and rising debt were weakening the country's medium-term outlook.