Caroline Diehl made her comments during a Charities Aid Foundation conference seminar entitled 'Turning heads or turning stomachs', in which she warned charities against resorting to graphic imagery.
A Media Trust survey of 20 creative agency directors revealed that three-quarters would urge charities to think again if they approached them with a brief to cause shock.
"They were all were very nervous about shock tactics," said Diehl. "They said the public was looking for a positive message and proof that their donations will be used in a particular way."
Diehl said she suspected that charities flouted Advertising Standards Authority guidelines because they thought they could get away with it.
"There is a perception in the regulatory body that charities think they can overstep the mark because they think they are doing good," she said.
Charities have been responsible for the two most complained-about adverts to the ASA in the past two years. Barnardo's Silver Spoon campaign, which showed cockroaches in children's mouths, provoked the biggest outcry in 2003, and a British Heart Foundation ad showing a woman with a plastic bag over her head caused outrage in 2002.
Diehl said uncomplicated factual adverts were generating the best response in the latest interactive TV campaigns on the Community Channel: "Charities that have simple, straightforward ads saying what they are doing with the money and what effect it has are getting the best response."
Although controversial campaigns could provide short-term fundraising and communications benefits, many donors did not respond favourably in the long term, said Diehl. She cited Oxfam and World Vision as charities that had rejected the hard-hitting approach.
John Grounds, director of communications at the NSPCC, which released an advert three years ago depicting a man inflicting horrendous abuse on a cartoon boy, said the issue was about responding appropriately.
"The NSPCC doesn't seek to use shock advertising," he said. "We are trying to communicate a message about a shocking thing that is happening in society."