Standard & Poor's has warned Britain that worse-than-expected economic growth or slow progress in fixing its budget deficit could cost the country its top-notch credit rating.

S&P affirmed the UK's AAA sovereign credit rating but kept the outlook as negative.

"The outlook remains negative, reflecting our view of at least a one-in-three chance that we could lower the ratings if the UK's economic and fiscal performances were to weaken beyond our current expectations," S&P said.

Britain lost its AAA rating from Moody's in February, an embarrassment for the Conservative-led government which had promised to protect the country's credit rating when it took power in 2010.

But slower-than-expected economic growth since then has meant the government is behind on its programme to return the country to fiscal health.

S&P said yesterday it expected the British economy to grow by an average of 1.6 per cent a year between 2013 and 2016, slightly slower than forecasts last month issued by the country's independent budget watchdog.

Fitch said in March it was likely to lower its AAA rating on Britain before the end of this month.-Reuters