Fifteen per cent of charities are doing work that does not meet their objectives, according to the Charity Commission.
The figure is based on findings from 860 review visits by the regulator over the past 18 months.
"It's a pretty horrendous figure," said Jonathan Brinsden, charity lawyer at Bircham Dyson Bell. "The risk these charities are exposing themselves to is a big one."
Brinsden warned that if a charity took on a project or service that was outside its objectives and made a loss, the commission could force its trustees to pay back the deficit.
The commission also revealed that 60 per cent of charities don't check if their trustees are eligible and that more than half have inadequate measures to deal with trustees' conflicts of interest.
The full findings will be published in spring, but Rosie Chapman, executive director of policy and effectiveness at the commission, will give a summary in her column in next week's issue of Third Sector.