Transgender girls have been banned from playing on female sports teams by a Republican governor in the US.

Alabama's Kay Ivey signed legislation restricting transgender students from participating in K-12 sports - those that take place during primary and secondary education.

The move will see Alabama become the latest conservative state to take such action against transgender girls.

Ms Ivey's office said in a statement that she had signed the bill saying a public K-12 school "may never allow a biological male to participate on a female team".

Spokeswoman Gina Maiola said she could not comment further on the decision.

Supporters of the bill, HB 391, insist transgender girls are born bigger and faster and have an unfair advantage in competition.

But critics say the bills are rooted in discrimination and fear, and breach the federal law barring sex discrimination in education.

"HB 391 is nothing more than a politically motivated bill designed to discriminate against an already vulnerable population," Human Rights Campaign President Alphonso David said.

"By signing this legislation, governor Ivey is forcefully excluding transgender children. Let's be clear here: transgender children are children.

"They deserve the same opportunity to learn valuable skills of teamwork, sportsmanship, and healthy competition with their peers."

The Alabama House voted 74-19 in favour of the bill, which was sponsored by Republican Representative Scott Stadthagen of Hartselle.

The Alabama Senate voted 25-5 for the legislation.

"I want to thank governor Ivey for her leadership and for protecting the rights of Alabama's female athletes," Mr Stadthagen said on Friday.

"Standing up for what is right is not always easy, but it is always the right thing to do."

In the Senate debate on the bill last week, Republican senator Garlan Gudger of Cullman said it was "unfair for biological males to compete and beat females in high school sports".

He said the bill is needed to protect the integrity of athletic programmes.

However, Republican legislators across the country have struggled to come up with actual instances in which a transgender girl's participation has caused a problem on a girls' sports team.

Other states with similar policies include Mississippi and Idaho - although one was vetoed by Kansas governor Laura Kelly, who said the Republican-backed measure was a jobs killer that harmed children.

Senate Minority Leader Bobby Singleton said the bill will give Alabama a "black eye" as the state tries to recruit industries and sporting events to the state.

"We are spending too much time on craziness like this," he said.