The collective pension deficits of Britain’s biggest companies have increased by £30 billion since the beginning of May, according to Towers Watson.

Total deficits in FTSE350 companies’ pension funds are estimated to have increased from £62 billion at the end of April to £92 billion by 16 May.

John Ball, head of UK Pensions at Towers Watson said: “Although the stock market has taken a dive, almost three-quarters of the growth in deficits is due to liabilities getting bigger rather than assets losing value.

“The latest stage of the eurozone crisis has pushed down interest rates on high quality corporate bonds as well as gilts, as investors look for safe havens. The lower these interest rates go, the bigger the pension liabilities in company accounts will look.”

During May, yields on corporate bonds have sunk to their lowest levels since accounting standards made them a key reference point for companies calculating pension numbers to put in their accounts. These yields are used to convert the pension payments that companies expect to make over several decades into a single liability number.

John Ball said: “Real interest rates are close to historic lows, especially now that inflation expectations have crept back up after softening earlier this month. An increase in pension deficits of £30bn for FTSE350 schemes in the space of a fortnight shows how easy it is for hard-earned profits to be offset by changes to the pension obligations on a company’s balance sheet when markets are volatile.

“Changing market conditions can present opportunities as well as threats. Companies and pension scheme trustees are increasingly thinking about how they might react to market movements to seize short-term opportunities and lock in future improvements in their funding position.”