Financial reserves released by general insurers in 2015 have been the highest in the past 30 years, according to a letter published by the Prudential Regulatory Authority (PRA).
Addressed to UK insurance CEOs, the letter said this was measured by the percentage of reserves based on a PRA analysis of regulatory returns.
The PRA's general insurance director Chris Moulder said: "Inevitably this raises the question as to whether these reserve releases are sustainable."
The PRA did not identify a single trend to explain the increase in reserve releases, but outlined a number of possible reasons.
These include: insurers' one-off exercises to clear out any excess reserves left in accounts from previous years; a movement more towards best estimate basis; pressure to maintain a certain level of profitability; and speeding up in the reporting and settlement of claims, reflecting improvements in claims processes.
The regulator acknowledged these factors could speed up claims settlements, but Moulder added: "We would also expect insurers to understand how much credit is being taken for these trends - noting that not all of these may yield the benefits anticipated."
The PRA also looked at how firms estimated future claims inflation assumptions implied by book reserves. In several cases, it found that the implied future claims inflation was lower than that indicated in the historic data.
In an extreme case, the regulator estimated the historic claims inflation to be 5% per year, but to obtain the insurer's booked reserves would imply an assumption of -2%.
"This would suggest that if the future trend is in fact in line with past inflation, booked reserves would need to be 25% higher than currently assumed," Moulder said.
He added: "We expect insurers to consider the impact of a range of inflationary assumptions so that boards are able to understand the sensitivities in this area."
The regulator will continue to look into the issue. For Lloyd's syndicates, the PRA also identified some common trends for specific lines but added Lloyd's would conduct their own review.