The total deficit for the final salary pension plans of the UK’s top 100 companies has shrunk to £21 billion over the past five years, according to a new report.

Actuaries at consultancy Deloitte estimate that 25 per cent of FTSE 100 companies now actually have a surplus in their schemes.

In the late 1990s onwards, the financial stability of many final salary schemes deteriorated and three years of falling stock markets between 2000 and 2003 coupled with the costs of rising life expectancy exacerbated the situation.

Gordon Brown’s decision to scrap pension tax credits is also estimated to have cost pensions’ billions.

However, improved stock market returns since 2003 as well as a falls in the price of bonds have helped push the deficit down.

But many firms have helped to cut their losses by closing their final salary schemes, which guarantee an income in retirement based on length of service.

Instead companies have switched their employees into less costly money-purchase or personal pension plans, which moves the investment risk on to their employees.

David Robbins of Deloitte said: “For the first time since 2001, we are starting to deal with schemes which have surpluses. Surprisingly, a surplus can be a headache for the company as it is near impossible for the employer to take a refund from a pension scheme. Many companies have now closed their schemes so may find it increasingly difficult to use up the surplus. The surplus could effectively become stranded”.

Deloitte argue that committing large cash sums up-front may not be the best option to fund pension deficits and advise companies to start looking at strategies that reduce the risk of a stranded surplus arising or defer the payment of contributions whilst still providing members with security for their pensions.

Aon, another consultancy pointed out that the resurgence of volatility in the stock market meant that weekly changes in the UK’s pension deficit of around £10 billion are commonplace – when global stock markets nosedived in late February the deficit rose by £11 billion in a single day, its highest one day rise since June 2001.

According to Deloitte in the last week of February, deficits rose by £20 billion as global stock markets fell.

Marcus Hurd of Aon said: "Most companies with March year ends are expected to report substantial improvements in their pension scheme deficit.

“Ten billion pound swings in the national deficit have occurred from one week to the next. Indeed, companies reporting a few weeks earlier would have reported losses over the year at a time when the national deficit was almost double its current value at £50bn.”

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