For more than 100 years, Britain's Girl Guides took an oath to "love God and serve the King/Queen."
But on Wednesday, the movement announced it would scrap its oath to God in an attempt to broaden its appeal and attract children from secular, nonbelieving families.
The controversial shakeup is seen by some as the biggest in the Girl Guides' history.
Beginning in September, all new members who make the promise to be good and useful citizens will pledge an oath to "be true to myself and develop my beliefs" and "to serve my Queen (Elizabeth II) and my country."
Andrea Minichiello Williams, CEO of Christian Concern, said the decision to remove God from the oath of loyalty was wrong.
"These values have their roots in a Christian outlook," she said. "Taking 'God' out of the promise denies the history and foundations of the movement without offering anything in its place, with the result that the organization will lose its distinctive ethos and end up meaning nothing."
But Chief Guide Gill Slocombe said the new promise was decided after a consultation involving almost 44,000 people.
She explained that the reference to God sometimes "discouraged some girls and volunteers from joining," adding that the new wording would help the organization "reach out to girls and women who might not have considered guiding before, so that even more girls can benefit from everything guiding can offer."
Graham Smith, campaign manager and CEO for the anti-monarchy organization Republic, said girls shouldn't pledge loyalty to God or the queen.
The Girl Guides in Australia dropped their allegiance to God and the queen last year, agreeing to be true to themselves and their communities instead.
Girl Scouts in the United States promise to "to serve God and my country." So do the Boy Scouts of America.
Girl-guiding attracts more than half a million members in Britain. There are an estimated 10 million members of the World Association of Girl Guides and Girl Scouts from 144 countries. They share similar principles and values, encouraging young women to grow into compassionate and capable citizens.
The new CEO, Julie Bentley, called the Girl Guides the "ultimate feminist organization."
The Girl Guides' Association was formed in 1910 under the leadership of Agnes Baden-Powell, the sister of Lord Robert Baden-Powell, who founded the Boy Scouts after spotting a need in the British Army for young men to learn the arts and crafts of "the bush" following his years in South Africa fighting the Boers (Afrikaners).
The Boy Scouts in Britain are also looking into the wording of their Oath of Loyalty, including how best to accommodate people who do not believe in God. A decision is expected next month.
After the World War II, the present queen and her late sister Princess Margaret were regularly seen in the blue uniform of the Girl Guides and were hailed as role models for teenagers.