Girls will no longer pledge their devotion to God when they join the Guides.
The organisation has announced it has dropped the phrase "to love my God" from its Promise after a consultation suggested that different words are needed to include non-religious youngsters and those of other faiths.
Instead, girls joining the organisation will now be asked to "be true to myself and develop my beliefs."
A mention of vowing to love or serve God has been included in the Promise since the Guides began in 1910.
Originally, new members were asked to "do my duty to God", and this was changed to "love my God" in 1994.
The move has been welcomed by secular groups who said it was a "hugely positive" development.
The new Promise also asks guides "to serve the Queen and my community" instead of "the Queen and my country".
Around 44,000 people responded to the consultation on changing the wording of the oath, Girlguiding said.
Chief Guide Gill Slocombe said: "Girlguiding believes passionately that girls need a space to explore their values and build the confidence to be true to themselves. Guiding has always been somewhere that all girls can develop their beliefs and moral framework, both inside and outside the context of a formal religion.
"However, we knew that some people found our Promise confusing on this point and that it discouraged some girls and volunteers from joining us. We hope that the new wording will help us 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.
"Guiding believes in having one Promise that is a clear statement of our core values for all our members to commit to. We hope that our new Promise will allow all girls - of all faiths and none - to understand and feel proud of their commitment."
It is the 11th time in the organisation's history that the Promise has been changed. The last time was in 1994.
The new Promise reads: "I promise that I will do my best: to be true to myself and develop my beliefs, to serve the Queen and my community, to help other people and to keep the (Brownie) Guide law."
Stephen Evans, campaigns manager at the National Secular Society said: "The introduction of one secular Promise for all is a hugely positive and welcome development.
"By omitting any explicit mention of God or religion the Guide Association has grasped the opportunity to make itself truly inclusive and relevant to the reality of 21st century Britain. The new secular promise can now be meaningful and relevant to all guides and potential leaders, whatever their beliefs - and sends a clear signal that Girlguiding is equally welcoming to all girls."
Andrew Copson, chief executive of the British Humanist Association said: "We wholeheartedly welcome the progressive step that Girlguiding have taken today of making their movement genuinely open to all, including the large number of girls and young women who don't believe in any god.
"We welcome the fact that the new Promise is about personal integrity and ongoing and active self-reflection, both of which sit well alongside a sense of responsibility to others and to the community. Unlike its predecessor, this is a Promise that is inclusive of all girls and young women whether religious or non-religious."
Girlguiding is a leading UK charity for girls and young women with 546,406 members. It runs the Rainbows which is for five to seven-year-olds, Brownies, for seven to 10-year-olds, Guides for 10 to 14-year-olds and the Senior Section for 14-25-year-olds.