Tens of thousands of girls cannot join the Guides because potential leaders are being put off by criminal records checks, the Children's Commissioner said yesterday.

Nearly 50,000 girls aged from 10 to 14 face severe delays in joining a patrol because red tape means there are not enough adults cleared to supervise them, said Professor Sir Al Aynsley-Green.

Other groups are understood to have raised concerns about criminal records checks, including St John Ambulance.

The Government is trying to clear the logjam by publishing guidance on when volunteer groups need to ensure staff have been vetted by the Criminal Records Bureau. The CRB helps organisations weed out people who might be unsuitable to work with children or other vulnerable members of society.

However, Sir Al claimed the requirement was acting as a disincentive to some adults.

He told MPs on the home affairs select committee: "There are 50,000 young girls who want to be Guides and they cannot be because adults don't want to do it.''

Whitehall sources said that Sir Al had been told by a number of organisations that potential volunteers viewed CRB checks as "too arduous''.

Earlier this month ministers admitted "potential volunteers can be put off if they were being asked to undergo a CRB check without good reason''. The Cabinet Office published guidance about when organisations need to carry out CRB checks on volunteers.

Phil Hope, the Cabinet Office minister, said: "It is a real waste if volunteers are being put off doing their hugely valuable work because checks are being carried out unnecessarily.

"Risk has to be managed properly and proportionately and I hope that this guidance will help organisations by giving them a clear step-by-step process to follow. I want to encourage more people to volunteer and will continue to tackle the barriers that prevent them.''

David Ruffley , the Conservatives' shadow home office spokesman, added: "The red tape that is clogging up the CRB's bureaucracy needs to be looked at urgently. CRB checks are very important but equally they must be carried out intelligently.''

A new Independent Safeguarding Authority is considering how to ensure that the correct people are checked by the CRB.

Liz Burnley, the Chief Guide, said last night that more leaders were "clearly needed'' with 48,000 girls waiting to join Britain's 500,000 guides. She said: "Girlguiding UK believes that there are many reasons why adults might think it difficult to volunteer, but to our knowledge the need for a CRB check is not a significant barrier.''

St John Ambulance declined to comment on the claims about CRB checks but a spokesman said a "nervousness'' among adults about working with children had contributed to a shortage of youth leaders.