The UK's top companies are collectively facing a pensions black hole of up to £100 billion.

Investment bank Dresdner Kleinwort Wasserstein (DrKW) estimates FT-SE 100 companies have pensions deficits adding up to between £50 billion and £100 billion.

The group blamed the problem on overly optimistic assumptions about the returns firms will get on equities, adding that these projected returns helped mask the shortfalls.

But it warned that firms may "sweep the issue under the carpet", as owning up to the overly optimistic assumptions would mean they would have to start reducing their pension fund deficits.

It added this would hit profits and could lead to firms reducing the dividends they paid.

The report said, on average, firms estimated returns on their pension fund assets would be 7.2 per cent a year, assuming they are invested equally in bonds and equities.

This means firms were expecting returns on equities of around 9.5 per cent a year, when the value of equities has actually fallen for the past three years.

The report came on the same day as thousands of Rolls-Royce workers faced the threat of higher pensions contributions after the engine maker revealed a £1.1 billion funding gap.

The group said it was in talks with staff on ways to reduce the deficit, which has ballooned because of stock market turmoil.

Rolls-Royce stressed it had no plans to scrap the final salary scheme but added options could include higher employee contributions or the curbing of some benefits, including those enjoyed by staff who retire early.