The voluntary sector's use of jargon and cliche is excluding and irritating the very people it is designed to target, according to environmental charity Encams.
Peter Gibson, media manager at the organisation, which runs the Keep Britain Tidy campaign, said jargon had seeped into charities' posters, personal letters and fundraising packs.
"Environmentalists use terms such as 'sustainable development' and 'Local Agenda 21'," he said. "No wonder the public thinks greens are hippies, loonies and anoraks who might be concerned about this planet, but live on another. Charities need to examine the language they use in their public messages very carefully."
Gibson said Encams had reviewed the way it communicated in order to remove jargon, because the right language was as important as targeting the right people in campaigning.
Its next campaign on fast food rubbish, which starts on 5 June, uses the slogan '40 years of dirt' to appeal to young male football fans.
"The phrase is designed to resonate with our target audience," he said.
"We know from research that those most likely to drop fast food rubbish are young men.
"If you're talking to people in the sector and they understand what you're talking about, that's fine. But if you use terms such as 'best practice' when you're communicating with the public, people are going to ask what you're going on about."
Dave Smith, a spokesman for the Campaign for Plain English, said: "The use of jargon in communicating with the public is a problem across all sectors. There is a particular problem with personal letters in which organisations use the jargon that's bandied about in the office."