A study conducted by the PRCA across 15 members of its public sector group also revealed the average time it took an agency to complete a public sector tender was 24.9 hours.
With each agency applying for an average of 8.3 public sector tenders a year, this means agencies could spend nearly nine 24-hour days on public sector pitching a year.
However, 30 per cent of such tenders are never awarded.
Agency bosses said public sector bodies often did not appreciate how time-consuming and costly it was to fill out briefs, including processes that seemed unnecessary.
Grayling UK public affairs MD Tanya Joseph said: 'They don't understand how much time it takes us to fill in these things. You have to do some research if you want the best.'
Flagship director of new business Julia Woodcock called for a database of information on agencies, to which all public sector bodies could have access.
Other complaints included bodies inviting too many people to pitch, with some lists including as many as 20. Finding tenders was also difficult, with briefs often being 'hidden on people's websites'.
