The Girl Scouts of the United States of America filed a trademark infringement lawsuit against the Boy Scouts of America on Tuesday, after the Boy Scouts decided to drop "Boy" from its namesake program and start welcoming older girls.

The lawsuit in Manhattan federal court is an attempt by the Girl Scouts, founded in 1912, to avert an erosion of its brand and membership as the Boy Scouts, founded two years earlier, tries to reverse its own decades-long membership decline.

The Girl Scouts claim they have been "marginalised" by the move, which they argue has sown confusion.

It was filed after the Boy Scouts, which accepts children 11 to 17 years old, said in May it would change its name to Scouts BSA in February 2019, and make girls eligible to earn its highest rank, Eagle Scout.

The Boy Scouts said in a statement it was reviewing the lawsuit.

"We applaud every organization that builds character and leadership in children, including the Girl Scouts of the USA, and believe that there is an opportunity for both organizations to serve girls and boys in our communities," it added.