The pension scheme deficits of the UK's top 350 public companies have widened further, according to the actuarial firm Mercer.
Rentokil plans to close its final salary pension scheme to all staff
The combined deficit of their pension funds rose from £75bn in 2004 to £93bn last year, Mercer said.
That was despite a rise in the value of the UK stock market, which boosted the assets of many schemes.
Mercer said the cost of paying pensions also rose because of lower returns on bonds and increased life expectancy.
"Favourable investment performance did little to dilute the value of pension scheme deficits in 2005," said Tim Keogh, worldwide partner at Mercer.
"Just as people have to pay more to trade up their house after a property boom, despite the value of their current home increasing, employers have to contribute larger cash sums to reduce their pension scheme deficits when all markets rise," he added.
The increased deficits calculated by Mercer highlight the continuing strain on company pension funds and the finances of the employers who run them.
The last decade has seen many private sector final-salary schemes close to new members.
Now there are fears that many employers will also start to move existing staff into less generous schemes which are cheaper to finance.
This week both the Co-operative Group and the retail conglomerate Arcadia outlined plans to save money by changing their pension schemes for current staff.
Last month, Rentokil became the first company in the FTSE 100 share index to say it wanted to close its final-salary scheme for all staff and not just new members.
The Pensions Regulator has made it clear that where deficits exist he expects pension fund trustees and employers to agree extra contributions to eradicate the deficit within 10 years.
The newly set-up Pension Protection Fund (PPF) will also impose an extra cost on pension funds. To finance the PPF the 8,000 private sector final-salary schemes it covers will have to pay nearly £600m in the next financial year.
Mercer said it suspected that in 2005 additional pension fund contributions by big firms were probably not much higher than the extra £5bn paid in during the previous year.
"Our experience suggests that many companies have waited to find out the cost of their PPF levy and the strength of the new funding regulations before they revise their contribution plans," said Mr Keogh.
"Despite some companies making substantial contributions in 2005, often to facilitate a major deal, we have yet to see the radical change in contribution strategy the Pensions Regulator is probably hoping for," he said.