The Boy Scouts of America lifted its outright ban on openly gay adult leaders and employees on Monday, rolling back a policy that has deeply divided the membership of the 105-year-old Texas-based organisation.

The new policy, which takes effect immediately, comes three years after it removed its prohibition on gay youth, but local Boy Scout units chartered by religious organisations will still be permitted to exclude gay adults from volunteering as den leaders, scoutmasters or camp counsellors.

The latest move was widely seen as being aimed at quelling a backlash against the Boy Scouts amid declining membership and the threat of litigation, while addressing concerns of religious institutions that account for about 70 per cent of the 100,000-plus Boy Scout units nationwide.

The rest are chartered to civic groups and educational organisations.

The Church of Jesus Christ of Latter-day Saints, the largest of all Boy Scout sponsors, said in a statement it was "deeply troubled" by the move and said the Mormons' "century-long association with Scouting will need to be examined".

The resolution implementing the change was backed by 79 per cent of the National Executive Board members voting and present on Monday, the Boy Scouts said. Its executive committee had unanimously recommended adoption of the new policy on July 13, citing a "sea change in the law with respect to gay rights".

The Boy Scouts' president, former US defence secretary Robert Gates, called for change in May, saying continuation of the blanket ban on gay Scout leaders was "unsustainable".

He repeated that assertion on Monday, saying efforts to keep the old policy intact would lead to "simultaneous legal battles in multiple jurisdictions and at staggering cost".

Moreover, a number of major corporate sponsors, such as Lockheed Martin and Intel in recent years dropped their support for the Boy Scouts in protest at policies they considered discriminatory.

Gates stressed the new policy enables religiously chartered Scouting units to "continue to use religious beliefs as a criterion for selecting adult leaders, including matters of sexuality".

However, no adult applying for a job as a paid employee or as a volunteer outside a local unit will be turned away on the basis of sexual orientation, according to the resolution.

The decision follows the landmark ruling in late June by the US Supreme Court legalising same-sex marriages nationwide.

The Boy Scouts lifted its ban on gay youth in 2013. The selection of Gates as president last year was seen as an opportunity to revisit the policy on adult leaders since he helped end the "Don't Ask, Don't Tell" policy that barred openly gay people from serving in the US military.

The Boy Scouts of America, whose stated mission is to prepare youth for life and leadership, has 2.5 million members between the ages of 7 and 21 and about 960,000 volunteers in local units, according to its website.

The anticipated end of the Boy Scouts ban has been welcomed by gay rights advocates and criticised by conservatives.

Zach Wahls, an Eagle Scout and executive director of Scouts for Equality, has labeled the ban a "towering example of explicit, institutional homophobia".

"While we still have some reservations about individual units discriminating against gay adults, we couldn't be more excited about the future of Scouting," he said on Monday. He called on those who left Scouting because of its previous policies to "rejoin the fold".

John Stemberger, chairman of the Christian youth outdoor program Trail Life USA, said on Friday that lifting the ban would be an affront to Christian morals and will make it "even more challenging for a church to integrate a [Boy Scouts] unit as part of a church's ministry offerings".