The Girl Guide Promise has been rewritten to remove mentions of God or country, in a move which is likely to outrage traditionalists.
The 103-year-old organisation has replaced the demand to "love God" with the promise to "be true to myself and develop my beliefs".
It will also ask girl guides "to serve the Queen and my community" instead of "the Queen and my country".
It is the eleventh amendment in the guide's history. The command to "love my God" was itself an amendment, made in 1994, to replace the old formulation of to "do my duty to God".
"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," said British Humanist Association chief executive Andrew Copson.
"Their initiative sends out a strong signal that young people who do not see themselves as belonging to any religion have values that are worthy of respect and should be explicitly welcomed."
Around 44,000 people responded to the consultation on the changing of the wording
"We hope that our new Promise will allow all girls - of all faiths and none - to understand and feel proud of their commitment," chief guide Gill Slocombe said.