The move is an embarrassment to the British government which promised to slash the deficit and protect the country's rating when it took power in 2010.
In a statement Fitch said: "Despite the loss of its 'AAA' status the UK's extremely strong credit profile is reflected in its 'AA+' rating and the stable outlook."
In February Moody's became the first agency to downgrade the UK and Reuters have quoted Standard & Poor's as having said there is at least a one-in-three chance it will follow suit.
