Fitch said the likelihood of a downgrade had increased after the economy's poor performance over the last year during which it has slipped back into recession.
The ratings agency pointed out that the UK had a debt mountain second only to the US among those countries with a AAA rating and was nearing the upper limit the agency set for the most creditworthy countries.
Fitch praised the UK for maintaining "a high-income, diversified and flexible economy as well as robust institutions and a high degree of political and social stability".
But it said a deterioration in the global economy and the government's struggle to balance the budget by 2015 had increased the likelihood of a downgrade. It warned against any plans to ease up on the government's austerity measures that could increase the current deficit.
"Global economic headwinds, including those blowing from the on-going eurozone crisis, have compounded the drag on UK growth from private sector deleveraging and fiscal consolidation as well as from depressed business and consumer confidence, weak investment, and constrained credit growth," it said.
"Fitch now expects the economy to contract by 0.3% in 2012 compared to an expectation of growth of 0.8% when the UK sovereign rating was last formally reviewed in March 2012. It added: "The weaker than anticipated economy is reflected in lower corporate tax returns and higher public sector net borrowing, which in the five months to August was £59bn compared to £48.4bn over the same period in 2011. In light of these developments, Fitch has updated and revised its medium-term fiscal projections for the UK."
The US was downgraded by rival ratings agency Standard & Poor's last year, but retained its AAA rating with Fitch and Moody's.
The UK is expected to grow only modestly next year, putting more pressure on the Treasury's finances. Low growth has dented tax receipts and increased social security payments in the first half of the current financial year and increased the government's debt mountain.
Fitch calculates that the UK's general government gross debt (GGGD) has reached 97%. A breach through the 100% barrier will trigger a downgrade, it said.
guardian.co.uk © Guardian News and Media Limited 2010
image: © bisgovuk