The U.K. faces a "significant" risk of losing its triple-A rating, as its economy has worsened since it was placed on negative outlook last year, David Riley, Fitch Ratings global managing director for sovereign ratings, told CNBC.
"We have had the U.K. on a negative outlook for almost 12 months and during that period, the economic and the fiscal outlook has actually deteriorated," Riley said on Monday.
The U.K. has retained its triple-A rating from all three major ratings agencies since the 1970s, including through the "winter of discontent" in 1978, the sterling's crash out of the Exchange Rate Mechanism in 1992, and the recent European financial crisis. However, it was placed on negative watch by both Moody's Investors Service and Fitch Ratings last year.
Riley said the U.K. gained from the exchange rate flexibility of having its own currency, and praised its "relatively credible" debt reduction strategy.
"But having a plan without growth is not really enough. I think all of us have to see some basis of economic recovery in the U.K., otherwise the pressures are going to build up in terms of the rating," he said.
Riley's concerns echo those of the International Monetary Fund (IMF)'s chief economist, Olivier Blanchard, who last week warned that the U.K.'s commitment to its austerity program might be strangling growth. However, the Chancellor of the Exchequer George Osborne defended the program while speaking to CNBC at the World Economic Forum last week in Davos, Switzerland.
"I do not think it is right to abandon a credible deficit plan... You cannot have a competitive, successful economy that people want to invest in if you cannot pay your way," Osborne said.
(Read More: UK's Osborne: Wrong to Abandon Austerity Plan)
On Friday, the U.K.'s Office for National Statistics reported gross domestic product (GDP) fell by 0.3 percent in the fourth quarter, with output from mining and manufacturing down 1.8 percent quarter-on-quarter.
The contraction spurred fears the U.K. could be heading for a triple-dip recession. The economy grew 0.9 percent in the third quarter 2012, but is seen shrinking again in first quarter 2013.
(Read More: UK GDP Drop Raises Risk of Triple-Dip Recession)
Kit Juckes, global head of foreign exchange strategy at Societe Generale, said a U.K. downgrade could push the pound lower.
"The U.K. is a small open economy with a current account deficit that relies on other people to come in. It is not like the dollar, where you can rely on other central banks around the world with pegged exchange rates to wade on in and buy your currency and bonds," Juckes told CNBC on Monday.
"Here, everyone else looks at the U.K. and says: 'Don't fancy that'. We have to wait for oligarchs to buy our football teams before we can come back."
Juckes forecast the U.K. could lose its triple-A rating in the first half of this year.
"In that time… we are going to try and make up our minds whether another period of negative growth is a triple-dip recession, a depression, a mini-depression, or what it really is, the economy going sideways for a really long time."
In a global asset report in November, HSBC forecast the U.K.'s debt-to-GDP ratio would reach 94.8 percent in 2013, the highest ratio of any country still rated triple-A by all three major ratings agencies. According to HSBC, Fitch has previously warned it might downgrade the U.K. if its debt ratio peaks above 100 percent, while Moody's said a downgrade is likely if its debt ratio does not look set to fall within the next three-to-four years.
-By CNBC's Katy Barnato