Pensions Ombudsman Paul Kenny says there was a 56% increase in his caseload in the first half of this year.
His office received 908 new complaints, compared with 582 in the same period last year.
The Pensions Ombudsman said his office was able to close 942 cases in 2009, more than double the amount a year earlier.
Mr Kenny said his office was now under 'fairly intense pressure, with 440 complaints under investigation at the moment. But he said a new system for managing cases - as well as an increased focus on mediation - had reduced the number of cases in hand.
Mr Kenny said that if the parties to a complaint were open to mediation, the case could be closed much more quickly than having to go to a final determination.
The ombudsman said he was not surprised at the large increase in complaints, as more people were reaching pension age and were increasingly vigilant on how their funds are invested.
