There are 51,000 children on a waiting list to become Scouts, Beavers, Cubs or Explorers, according to the organisation behind the movement.
The Scout Association, which is open to girls and boys between six and 18, blamed a shortage of volunteer leaders.
It said record numbers of adults were members but that volunteers had limited amounts of time.
Chief Scout Bear Grylls said it was a "challenge" to recruit more people.
"We've got 51,000 young people wanting to join and benefit from what scouting offers," he said.
"Volunteering changes us all for the better. Please join me."
There are 457,000 Scouts, including Beavers (aged six to eight), Cubs (eight to 10), Scouts (10 to 14) and Explorers (14 to 18).
Although 154,000 adults currently volunteer, which the Scouts said was an all-time high, people are committing less time than before.
About 17,000 more volunteers are needed, the Scout Association said. Leaders are at their scarcest in Surrey, Devon and Merseyside.
'You see them grow'
Hannah Kentish, 24, from Greenwich, in south-east London, has been a volunteer with the scouts for six years at a Cub pack in Erith and a Scout group in Welling.
"When I joined the Scouts, my confidence grew massively. I was selective mute and not comfortable in groups of people my own age. But my confidence grew and I wanted to give that confidence to others who were also shy.
"I've always found volunteering flexible. I began at 18 doing a couple of hours every Monday after school. While at university, I only really helped during the holidays, and would go to camps. Afterwards while I was finding my feet in a new job, I volunteered about once a month.
"My fondest memory is when I took my cubs to Paris.
"For some, it was the first time they had ever been abroad so they were very excited about going on a ferry and getting their passport. It was the first chance for them to take responsibility. You see them grow as they learn about a new culture and visit the Eiffel Tower, that they had only ever seen in pictures before."
The organisation said volunteers typically commit to attending sessions on a regular basis and will plan what to do during meetings, as well as undertake training.
Those volunteering with older children, like the Scouts, may also go on trips or expeditions over the weekend.
Tim Kidd, UK chief commissioner at the Scout Association, said the organisation was making it easier for those with limited time to join by being flexible about a range of roles, including group leaders, administrative and trustee positions.