Atheist children are being excluded from the Scouts, the National Secular Society (NSS) has warned in a letter to Chief Scout Bear Grylls.
The NSS, which aims to restrict the role of religion in public life, says the scout promise, which refers to God, puts non-believers off joining.
And it asks why adults have to profess a belief in God to become leaders.
The Scout Association says membership continues to grow and the movement is more popular than ever.
The NSS says it is frequently contacted by concerned parents whose children have been told they can't become scouts unless they recite the whole of the obligatory scout promise which includes the line: "To do my duty to God and to the Queen."
"Scouting has something to offer everyone, no matter your religion, ethnicity or belief, and I'm so proud that we offer an environment for people of all backgrounds to come together and enjoy themselves."
In response, the NSS has now written to Mr Grylls, urging him to support the introduction of a secular version of the scout promise.
They say that, unless this happens, atheist children will be excluded or will have to lie in order to join the organisation.
The NSS were unable to provide an exact figure for the number of complaints they have received.
The Scout Association told the BBC that the scout promise is an integral part of the worldwide scouting movement.
As a membership organisation, they say they are entitled to set certain criteria for joining.
Spokesman Simon Carter said the association was aware of the views of the NSS but did not intend to change the wording of the promise:
"We will continue to offer adventure and development opportunities to our membership of over 500,000 adults and young people which is steadily growing," he added.
The NSS also says atheist adults are prevented from becoming scout leaders, even if they were members as children.
The Scout Association says it remains popular with membership numbers increasing, especially among girls.
There has also been a rising interest in scouting among members of the Muslim community.
Up to 30,000 young people are on waiting lists hoping to join the Scouts, and the association has appealed in the past for more adult volunteers to come forward.