By Andrew Verity BBC Personal Finance Correspondent

Firms say pension schemes are 'long-term creatures'

A recent study put this pension gap, across all companies, at more than �90bn.

In some individual companies - such as British Airways or Rolls Royce - the estimated size of the pension deficit is bigger than the market value of the company.

And on average, the deficits are worth 97% of what the companies make each year in operating profits, according to Credit Suisse First Boston.

Five years ago, very few companies had a pension deficit of any size.

Slumping share prices

In fact, many had such a healthy surplus that they stopped paying into the schemes, taking so-called "contribution holidays" - and saving some �30bn in the process.

The surpluses arose because they had most of their members' pension money in shares - typically, 70% of it.

But after three years of slumping share prices, that investment is having the opposite effect.

Instead of surpluses, nine out of ten companies are running deficits.

Companies such as BT - which has a deficit running into billions of pounds - say they can manage their deficits because pension schemes are long-term creatures.

Market rebound

Some of the pensions - such as those for workers now in their twenties - will not have to be paid for 40 years.

A growing chorus of analysts and investment experts believe the schemes - and the trustees who run them - are taking unacceptable risks

Large pension schemes say they can afford to wait for the stock market to bounce back.

Schemes such as BT's have kept most of their money in shares so they will be in a position to benefit from a stock market recovery.

They also believe that in the long-run, shares will make more money than other forms of investment.

So short-term fluctuations in share prices should be ironed out over the long-term.

Game of two halves?

As BT's finance director Ian Livingstone has explained: "What's important in determining the health of the fund is the long-term rate of return of the assets it holds.

"If the share prices go up or go down today or tomorrow, that doesn't really reflect that long-term return."

However, a growing chorus of analysts and investment experts believe the schemes - and the trustees who run them - are taking unacceptable risks.

Pension deficits, according to them, are "a game of two halves" - the assets, and the liabilities.

Because most of the assets are shares, the value of the assets fluctuates every day with share prices.

Promises, promises

But the liabilities are the pensions that have to be paid.

How much will your pension be worth?

They fluctuate not with share prices but with other factors - such as the cost of financing the actual incomes paid out to pensioners.

To pay a pension to a pensioner, the company pension scheme will typically buy a bond, paying an interest rate that will give enough income to pay the pension.

If the scheme has enough bonds to cover its pension promises, it knows it can pay what it owes to pensioners - no matter what happens to the stock market.

This is known as "matching" assets to liabilities.

Bonds or shares?

But that puts some pension schemes in a quandary.

If they have a lot of pensioners drawing their pension, and not very many working members paying in to it, they will need a lot of bonds.

Companies in that position - such as BT or Rolls Royce - feel that would disadvantage their members.

In BT's case, it has a lot of pensioners and fewer people in its workforce.

So to be properly "matched" 70% of its money would have to go into bonds, rather than shares.

'Mismatching'

Finance directors object to this because they believe shares will always make more money than bonds in the long term.

So instead they take a risk - "mismatching" their assets and the liabilities.

In other words, they keep money in shares when they could be surer of paying their pensions with bonds.

Analysts such as John Ralfe, former corporate finance director of Boots, say that "mismatch" is no longer acceptable.

Staff have to rely on the company staying in business for decades into the future.

Big gamble

If at any point the company goes bust and it has too much money in shares then it will no longer be able to pay the pensions it has promised.

He also questions whether it is always true that shares make more money than bonds.

Perhaps, he argues, it is merely that that has been true over the last 40 years.

But that does not mean it will always be so.

And if it is not, company pension schemes are taking a big bet with their members' pension money.