George Osborne was served notice on Friday night that the UK's AAA credit status will be downgraded for a second time when ratings agency Fitch said Britain's lack of growth and rising debt mountain meant there was a "heightened probability of a downgrade in the near term".

Delivering its verdict on this week's budget, Fitch said it was writing a report, due in April, that analysts said is almost certain to strip the UK of the second of its three AAA ratings.

Fitch said the notice of a possible downgrade reflected "the latest economic and fiscal forecasts published by the Office for Budget Responsibility (OBR) that indicate that UK government debt will peak later and at a higher level than previously expected by Fitch".

The government will find out in April whether the economy suffered an unprecedented triple-dip recession. In the last three months of 2012 the economy shrank 0.3%. A second straight quarter of negative growth could undermine faltering business confidence and choke off the first signs of a recovery.

Figures from the Office for National Statistics are expected to show the UK narrowly avoided a third contraction since the banking crash of 2008, but recent surveys have shown key industries, including manufacturing and construction, slipping back.

Several analysts said even without a triple-dip recession the worsening economic situation made further credit downgrades inevitable.

In documents that accompanied the budget, the OBR said that a lack of growth and lower than expected tax receipts had worsened the economic outlook and delayed the recovery until after the next election. It halved the forecast for growth this year to 0.6% and said the debt-to-GDP ratio would peak at 85%.

But Fitch said a tougher measure of the UK's debt burden showed the debt pile would exceed 100% of annual national income before it begins to fall, which makes situation more perilous.

While it credited the UK with having a "high-income, diversified and flexible economy", it said the last four years of low growth had damaged the economy.

It said: "The persistently weak performance of UK growth, in part due to European growth, has increased uncertainty around the UK's potential output and longer-term trend rate of growth with significant implications for public finances."

Last month Moody's stunned the chancellor when it became the first agency to kick Britain out of the exclusive AAA club, which still includes Germany, Canada and Switzerland.

The Treasury has argued that the downgrade shows the UK needs to cut public expenditure to get on top of its debts.

Labour responded that the downgrades were the result of an austerity-first policy that failed to generate growth.

Chris Leslie, the shadow Treasury minister, said: "This is yet another blow to a downgraded chancellor who made keeping the confidence of the credit rating agencies the No 1 test of his economic policy. What really matters are the economic realities which Fitch are responding to including, as their statement says, 'the persistently weak performance of UK growth'.

"George Osborne's plan has catastrophically failed on growth, living standards and the deficit. But instead of a change of direction and action to kickstart the flatlining economy all we got this week was more of the same failing policies." He said the budget would "be remembered as a wasted chance to change course before it was too late."