US - S&P1500 pension plans are carrying a shortfall estimated at US$409bn - a deficit which could force companies to re-evaluate existing provision, Mercer says.
The consultant also found the funding status for pension funds had fallen to 75pc at the end of 2008 from 104pc a year earlier.
Hartshorn said such a fall was "unprecedented" in at least the past decade.
He added: "The deficits are going to cause a number of companies to take a step back and think about whether what they are offering is viable."
Mercer estimated pension expenses were likely to increase to an estimated $70bn in 2009, from $10bn in 2008 - forcing plans to either raise cash or invest their way out of a deficit.
Hartshorn noted raising cash in a cash-strapped market could be challenging - and said scheme managers would need to decide whether to funnel the money into the deficits or into other parts of the business.
Mercer said pension schemes had been hit by falls in the equity market and a fall in bond yields at the end of the year.
It said the S&P500 index alone dropped nearly 40pc in 2008 and noted bond yields - increases in which reduce the value of scheme liabilities - fell back from 8.5pc in October 2008 to 6.3pc at the end of the year.
Mercer said some plans could choose to partially opt-out of the volatile markets and invest more heavily in long-dated debt.
Mercer financial strategy group principal David Kelly said, while investing in longer-dated bonds could lock in deficits, some plans could prefer the stability of steady contributions into the plan to make up the deficit, than to continue to deal with the uncertainty of the equity markets.
But he said others would prefer to stay the course rather than sell their equity investments at a loss.
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