Ratings agency Fitch has stripped the UK of its AAA rating due to its "weaker economic and fiscal outlook".
The agency placed the UK on an AA+ rating, following Moody's downgrade of UK debt in February.
Fitch said: "The downgrade of the UK's sovereign ratings primarily reflects a weaker economic and fiscal outlook and hence the upward revision to Fitch's medium-term projections for UK budget deficits and government debt."
The downgrade will place further pressure on the Government ahead of next week's first quarter GDP figures, which will reveal whether Britain has managed to avoid an unprecedented triple-dip recession.
The agency now expects Government debt to peak at 101% of GDP in 2015-16, only declining gradually in 2017-18. That is worse than its previous forecast of debt peaking at 97% of GDP and declining in 2016-17.
Fitch, which waited until stock markets had closed before announcing the downgrade, had already warned that Government failure to stabilise debt below 100% of GDP and set it on a firm downward path would trigger a downgrade.
It added: "Despite the UK's strong fiscal financing flexibility underpinned by its own currency with reserve currency status and the long average maturity of public debt, the fiscal space to absorb further adverse economic and financial shocks is no longer consistent with an AAA rating."
Fitch slashed the UK's growth forecast to 0.8% this year, from its earlier expectation of 1.5%. Next year it expects the UK economy to grow by 1.8%, down from its previous 2% forecast.
Earlier this week the International Monetary Fund also cut the UK's growth forecast growth from 1% to 0.7% this year and 2014's projection from 1.9% to 1.5%, noting the recovery was "progressing slowly".
Economists largely expect the UK to eke out growth in the first three months of the year, after contracting by 0.3% in the final quarter of 2013. First quarter GDP figures are released on Thursday by the Office for National Statistics.