Multi-million pound black holes have appeared in staff pension schemes because of poor investment returns.
The pension scheme for Aon subsidiary Minet has a £32.2m deficit which will take five years to correct, according to documents seen by Insurance Times.
Members hoping to retire early or transfer their benefits could receive lower payments than they had expected.
The revelation came after reinsurance broker Bradstock revealed a £14m deficit in its pension fund.
The Aon Minet scheme has just 402 members still paying in but 624 already receiving payments and 1,600 waiting to receive payments.
It has now stopped taking new members.
The valuation was done using information as at 1 April 2000, before the stock market slump following the US terrorist attacks.
Aon has agreed to top up the scheme.
It will make stepped contributions rising to 25.2% of pensionable pay and £6m a year, rising each year by 5.8%.
The top-up payments should clear the deficit by 31 May 2007.
Tunbridge Wells-based independent financial advisers Newman Houghton & Co employee benefit specialist David Brunning said: "The scheme has problems.
"They are obviously underfunded. The 25.2% is way up the top end of the normal scale. Adding in £6m shows the scheme is in trouble."
He said anybody wanting to retire early could face lower benefits than they had expected.
Anybody wanting to transfer to an alternative scheme faced the likelihood of disappointing benefits.
The members could be asked to make extra contributions to help the scheme.
A spokesman for Aon was unable to say to what proportion of its commitments the scheme was funded.
"The scheme isn't in trouble. Members should not be worried."
Reinsurance broker Bradstock, said this week it would not be able to make up a deficit in its pension scheme from foreseeable trading profits.
The revelation came in its financial results for the year to 30 September 2001. The deficit was £14m as at the end of September last year.
Bradstock has proposed a solution to the scheme's trustees.
In a separate development, Cox wrote to staff paying into its Christopherson scheme last week telling them the scheme would be closed and they would be offered membership of an alternative plan.
The company denied the move was driven by a need to save money.
But Brunning said: "The employer is no longer prepared to underwrite the blank cheque of a final pension scheme.
"It's a very important change for members."
Under the Christopherson final salary scheme, staff could expect fixed pension payments based on their final salary.
Under the alternative offer, the Cox Group Personal Pension Scheme, the company still contributes, but the amount received by pensioners is not guaranteed.
It offers the employer greater flexibility on funding and ability to change contribution levels.
Cox's finance director Richard Brewster said: "It's cost neutral.
"We reckon the funding rate isn't much different. It's the risk we are moving away from."
Brewster said the scheme did not have a significant deficit.
The scheme has about 600 members, including 227 active members. The rest are either pensioners or waiting to receive benefits.