The Abacus consultancy director David Ferguson questioned the stated cost of smoothing, which Pru chief executive Mark Wood admitted is around a 0.75 per cent annual management charge.
Ferguson queried where the fund's capital for new investment and offers such as extra allocations came from.
He said, with returns under pressure, costs such as this would be difficult to sustain. He also asked Wood how it was possible to trust the management of with-profits, given the performance of many funds in recent years.
He questioned how advisers and savers are supposed to identify which with-profits offices are strong, citing the example of Standard Life, which he said would have been on everyone's list of strong offices a few years ago.
He said: “I agree the principle of with-profits is very strong and the characteristics are very well appreciated – my concern is mostly around how we judge what is a good with-profits policy.
“My big concern is why should we trust product managers now to manage it any better than we have the last three or five years?”