The charge is around a third of Royal Mail's profits last year.
Postcomm said it was to investigate reasons for "the recent collapse in Royal Mail's quality of service", which could cost Royal Mail £80m in compensation to business customers.
Royal Mail chairman Allan Leighton said that he and other executives would defer all or part of their bonuses until service levels were resumed to levels set by Postcomm. He said that targets had been missed because of unofficial action last autumn.
Royal Mail returned £220m profit for the year ending March 31 2004. It is believed that most of this can be attributed to price increases on first and second class stamps last May. Turnover rose 4% to £8.6bn.
Last year, Postcomm fined Royal Mail a record £7.5m. This was before the new compensation system was introduced.
Royal Mail announced that it would cut its advertising budget by more than 40% to £12m and has postponed marketing activity as part of a major cost-cutting exercise.
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