Mentoring charity Chance UK is losing 50 per cent of its volunteers because Criminal Records Bureau checks in London are taking up to five months to come back.
The charity requires enhanced CRB checks on all would-be volunteers before they can start because they work with children.
Gracia McGrath, chief executive of Chance UK, explained: "We ask our mentors for a commitment of a year, but if the checks take five months they often lose their impetus."
McGrath said that the problem seemed to be confined to London, and that checks outside the capital seemed to take a month on average.
A CRB spokesman said: "Demand for our services has increased - last year we carried out 2.5 million checks, and this year we have done 2.9 million.
As 15 per cent of all checks are made in London, it has been taking longer than we would like.
"We are working with the police to improve the situation by putting in place new resources."