Mississippi’s governor has signed the US’s tightest abortion restrictions into law.

Republican governor Phil Bryant signed House Bill 1510 on Monday afternoon. It becomes law immediately and bans most abortions after 15 weeks’ gestation.

Mr Bryant has frequently said he wants Mississippi to be the “safest place in America for an unborn child”.

The law’s only exceptions are if a foetus has health problems making it “incompatible with life” outside of the womb at full term, or if a pregnant woman’s life or a “major bodily function” is threatened by pregnancy.

Pregnancies resulting from rape and incest are not exempted.

Pro-choice advocates are calling the law unconstitutional because it limits abortion before foetuses can live outside the womb.

The owner of Mississippi’s only abortion clinic opposes the law and has pledged to sue.

Mississippi had previously tied with North Carolina for the nation’s strictest abortion limits at 20 weeks.

Both states count pregnancy as beginning on the first day of a woman’s previous menstrual period. That means the restrictions kick in about two weeks before those of states whose 20-week bans begin at conception.

“We certainly think this bill is unconstitutional,” said Katherine Klein, equality advocacy co-ordinator for the American Civil Liberties Union of Mississippi.

“The 15-week marker has no bearing in science. It’s just completely unfounded and a court has never upheld anything under the 20-week viability marker.”

The bill was drafted with the assistance of conservative groups including the Mississippi Centre for Public Policy and the Alliance Defending Freedom.

“We’re thrilled that Mississippi lawmakers are taking a step to protect the basic right to life, as well as protecting maternal health,” said Jameson Taylor, acting president of the Mississippi Centre for Public Policy.

Both Republican-controlled chambers passed the bill overwhelmingly in early March, by a vote of 35-14 in the Senate and 76-34 in the House.

The US Senate failed to pass a 20-week abortion ban bill in January. With 60 Yes votes required to advance, the bill failed on a 51-46 vote.