The regulator announced that it intended to appoint "an independent person of stature" to carry out the inquiry. A spokesman said the PIA took the leak "extremely seriously".
The figures, presented to a PIA board meeting earlier this month, show that redress has been offered in barely 2,500 cases, out of 360,000 identified as priorities.
However, the PIA's plans for a mole-hunt were attacked by consumer groups, who claimed its time would be better spent ensuring that victims of the pension transfer scandal received swift redress.
In addition one senior regulator, who declined to be named, said he was "baffled" by the move: "It could be argued that criticism is acceptable but leakage is not. But how do you criticise anything if you don't have the figures first?"
The PIA announcement came as new evidence mounted of more delays faced by those wrongly advised to buy a personal pension and transfer occupational scheme funds into it.
Sources said a decision by the Securities and Investments Board, the senior City regulator, to try to kick-start the faltering review process had been caused by new figures showing very low response rates to insurers' inquiries by their clients.
In the document obtained by the Independent earlier this week, this varied between more than 75 per cent for Barclays Life to less than 50 per cent for Norwich Union.
It is now believed that these figures relate to the first letter sent to clients, in which they are simply asked to indicate their willingness to take part in the review. A second mailing, which contains a detailed questionnaire for individuals to complete, is said to be achieving a far lower response rate.
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