The Guide Association said yesterday that "lots" of girls were on waiting lists to join their local groups, but were being turned away because there was no one to lead them.
"One of the problems is social change," said the association's spokeswoman, Jackie Bennett Shaw. "Many young women have to juggle family and their career - that leaves little time for voluntary activities.
"Social pressures on women are making it increasingly difficult to recruit leaders and that means we can't expand the work programme. "We find that the demand from girls far exceeds the number of leaders we have."
But she said they did not like advertising for new leaders for fear of attracting the wrong type of person.
"Advertising, we find from practical experience, is not particularly beneficial. The most effective way of recruiting is by personal referral and contacts. It is a youth organisation, it is a very responsible position, so you have to be very careful who you take."
One group facing a crisis is the 4th Littleport Brownies in Cambridgeshire, which has 20 girls and is in search of a new leader - just a few weeks after it began. Elaine Ellis, 42, who runs a neighbouring group, said: "What tends to happen at the moment is we rely on the goodwill of one or two individuals who struggle from year to year. You kind of rock from one crisis to another," she added.
National membership has fallen by 64,000 in the last 12 years. In 1983 there were about 814,000 Guides and Brownies - adults and girls. Now there are 750,000.
Church leaders said yesterday that the movement faced increasing competition from other pastimes. The Rev Simon Baker has seen a low turnout of Guides for the last decade at his parish of Shinfield, in Berkshire.
"There are many pressures in modern life for young people and joining an organisation that is uniformed and to which you have to go every week is very different from a lot of freedoms young people enjoy these days. We live in an age where young people find commitment difficult," he said.
And in the age of shopping malls and computer games, the single-sex uniformed organisation is often seen as quaint and old-fashioned. However, it insists it is moving with the times.
Pauline Ashton, executive director of the Girl Guides' Scottish organisation, said yesterday: "In Scotland, Guides are the largest youth organisation and we are still quite a force to be reckoned with.
"The girls and young women get a lot of personal development and the opportunity to do lots of interesting things - and that goes for the adult leaders, too."