Out of more than 350 consultancies surveyed by research company Question & Retain, 31 per cent agreed that over-servicing means the PR business model is not working well.
Just 3 per cent disagreed outright, while 40 per cent said it was a challenge to measure time effectively without agreeing it was a drain on profits.
Annabel Dunstan, co-founder of Q&R, said over-servicing – where agencies do more work than a client is paying for – was "endemic in the PR industry".
One of the agency heads who responded to the survey said clients’ demands diluted the financial rewards of running an agency.
The agency head said: "Clients expect, as they are paying you, that you should be keen to do more work and be happy about it. It’s a battle, particularly when you start a piece of work and then, for whatever reason, it comes to an end halfway through and then it is expected to be free.
"It’s hard to make any decent money in PR, it’s too resource-consuming."
However, another significant segment of the respondents – 26 per cent – were more resigned to living with over-servicing, describing it as "part of what you have to do to manage profit and loss".
In the words of another agency head: "We consistently over-service, but regard this as an investment in securing our next job from the client, which is cheaper than pitching for new business."