Fewer than a quarter of charities in England and Wales carry out background checks on prospective trustees, according to a new report.
Even 56 per cent of the largest organisations fail to check whether applicants are legally disqualified from taking up the role, the Charity Commission says.
The regulator ' s report, Start As You Mean To Go On, highlights a " patchy" understanding of the law surrounding disqualification.
It states: "It is disappointing that only 23 per cent of charities responding to our survey said they carry out checks on the eligibility of prospective trustees to serve on the board."
In some cases failure to follow these procedures has " embarrassed" the trustees and the charities concerned, the report says.
It adds: "In extreme cases, the charity's good name, the welfare of its users or even its existence could be at risk."
Three quarters of the 1,487 charities surveyed did not carry out checks on new trustees, compared to 23 per cent who did and two per cent for whom no information was available.
Organisations whose trustees have direct contact with " vulnerable" users, such as children or the mentally ill, are required to make Criminal Records Bureau checks before appointments are made.
In other cases, eligibility checks may simply mean asking new trustees to sign a declaration stating that they have not been legally disqualified from taking up the role.
Neville Brownlee, head of charity effectiveness at the Charity Commission, said he was satisfied charities were carrying out Criminal Records Bureau checks where necessary.
He said failure to do so was rooted in a lack of understanding of the law banning people who are bankrupt, or who have been convicted of an offence involving dishonesty or deception.
There are an estimated 890,000 charity trustees in England and Wales, the majority of whom carry out their roles free of charge except for expenses.