Pension schemes have been asked to consider whether they are being too generous when offering lump sums to people thinking about cashing out of "defined benefit" retirement schemes.
It comes amid concern that overly generous payouts could damage the remaining funds.
The Pensions Regulator wrote to 14 schemes earlier this year, encouraging them to consider making reductions.
A record £21bn flowed out of defined schemes in the year to March.
Defined benefit schemes promise people a certain level of income when they retire, such as final salary pensions, and are often described as "gold-plated".
However, in recent years firms have been offering people large cash sums to transfer out, as it gets more expensive to cover their obligations.
The letter was obtained by Royal London, one of the UK's largest pension fund managers, following a freedom of information request.
Sir Steve Webb, director of policy at Royal London, said people were routinely offered 25 to 30 times their annual pension as a lump sum transfer value - but it could be as much as 40 times.
This could mean that for a £10,000-per-year pension, someone could be offered anywhere between £250,000 and £400,000.
The former pensions minister said such generous payouts might pose a risk if the pension scheme was in deficit, or if the employer faced financial difficulties.
If large numbers of workers transferred out, he said, it could worsen the scheme's funding position and put other member's pots in jeopardy.
"I would hope that well-run pension schemes would be taking expert advice when deciding how much to offer to members wishing to transfer out," Sir Steve said.
"But the regulator's letter is a helpful reminder to all schemes that they need to be fair not only to those transferring out but also those left behind, especially where the scheme in question is in deficit."
In its letter, the Pensions Regulator said that transfers presented "certain risks" to members as they did not promise a guaranteed stream of income.
It also told the schemes, which have not been named, that it was in the "best financial interests" of most members to stick with their defined benefit schemes.
The costs of defined benefit schemes have risen as people live longer, and companies now tend to offer employees less generous defined contribution schemes.
However, large numbers of current and former workers still rely on them.
A spokesman for the Pensions Regulator said: "Our primary concern is that defined benefit scheme members requesting a cash equivalent transfer value have all the information they need to make an informed decision about what is in their best interests.
"This includes understanding the fees that are charged under any new pension arrangement as these can make a significant difference to the value of the fund.
"As a result, we are working closely with the Financial Conduct Authority and The Pensions Advisory Service to provide an increased level of support to trustees and scheme members where there is uncertainty around the future of a defined benefit pension scheme.
"This includes providing letters for trustees to send members alerting them to the risks of transferring and giving practical information."