The 90-year tradition of Scout Jamborees in the UK is under threat because of new Government vetting rules, it was revealed today.
As the first phase of the controversial anti-paedophile scheme starts, the Scout Association has warned it could mean major gatherings of packs from around the world are cancelled.
Organising criminal record and other checks on thousands of foreign Scout leaders was "just not possible", a spokesman said.
Volunteers who fail to register face the prospect of criminal prosecution and a fine of up to £5,000.
Simon Carter, Scout Association spokesman, said: "When we hold big international jamborees we rely on adults from other parts of the world coming in and staffing these events.
"The rules for checking people out suggest that if they were to come along and do intensive activity they would have to be checked.
"Clearly we cannot do that, it's just not possible."
The association has written to Children's Secretary Ed Balls asking him to allow voluntary groups greater flexibility within the rules of the scheme.
As well as concerns about major events, it warned parents who wanted to try out volunteering with their son or daughter's pack could be deterred by "bureaucratic and difficult" procedures.
Mr Carter said adults who were supervised constantly while in contact with children should be exempted from the rules for a short period of time.
The regulations made it "significantly harder for people to volunteer," he said.
From today employers and regulators have a legal duty to pass on concerns about anyone they think could be a danger to children or vulnerable adults.
The new Independent Safeguarding Authority will use the "soft intelligence" to rule on who should be barred from sensitive jobs.
New criminal offences mean employers who knowingly give jobs to dangerous individuals face up to five years in prison, as do barred individuals who apply for posts they are banned from holding.
A wider range of jobs, including every doctor, nurse, prison officer and school governor will have to be cleared.
It is estimated around one in four adults could eventually be registered with the ISA, which begins taking applications from new workers in July next year.
Amid public outrage last month, Mr Balls ordered a review of the scope of the scheme.
Officials emphasised volunteers' applications were free and would be processed within seven days.
A spokesperson for the Department for Children, Schools and Families said: "We recognise the importance of children attending and participating in international events such as jamborees organised by the Scout Association, and would not wish the Vetting and Barring Scheme to act as a deterrent to these events going ahead.
"While the fine detail is yet to be finalised, we are working in collaboration with the Scout Association and others to ensure that the practical operation of the scheme does not impede the operation of these events."
Events involving 10,000 Scouts and around 2,000 volunteers from dozens of countries take place every year.
In 2007 the World Scout Jamboree was held in Chelmsford, Essex, involving 40,000 Scouts and around 12,000 adult volunteers.
Lord Baden-Powell, the founder of the Scout movement, hosted the first Jamboree in London in the summer of 1920, involving 8,000 Scouts from 34 countries.