Some 147,000 over-55s would face charges of more than 5% if they transferred their pot to take advantage of retirement flexibilities introduced in April, according to the Financial Conduct Authority (FCA).
In a letter to the Treasury, the watchdog said most retirees would need to transfer to get the full range of retirement income options, particularly if they were interested in drawdown.
It said the majority of those eligible to access their pots (84%) would not face a charge to exit their current scheme, while 9% would have to pay up to 2% of their pot.
Some 4% would face charges of 2-5% while 3-4% would have to pay exit fees in excess of 5%.
The FCA said its analysis, produced to inform the Treasury's consultation on exit charges, represented a snapshot of the exit charges faced by savers at the moment.
It found that 200,000 pension policies were accessed in the three months after April.
While 80-90% of people could theoretically access their pension through drawdown or uncrystallised fund pension lump sums without transferring providers, the FCA found most had to switch to a new contract within their existing provider to access their money in their preferred way.
It said: "While firms are continuing to develop new options for their customers, with many expected to be available within the next six months, we expect many consumers will continue to need to transfer to a new contract when accessing their pension savings."
The regulator found most providers required consumers transferring into their pension products from another provider to take advice which goes beyond statutory requirements, particularly among the largest firms.
"Many consumers seeking to transfer their defined contribution pension would find that their transfers were not accepted by a significant number of providers, particularly where they wished to transfer safeguarded pensions," it said.
Similarly, only a quarter of providers and only 15% of the largest 15 said that they would accept transfers from Defined Benefit (DB) pensions in all circumstances.
Transfer times were found to be 16 days on average, a figure the FCA described as "low". It praised the Origo system for helping to speed the process up - transfers through the industry platform took six days on average.
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