Idaho has become the first US state to enact a law modelled on a Texas statute banning abortions after about six weeks of pregnancy, even though the governor who signed it shared concerns.

Republican Brad Little signed into law the measure that allows people who would have been family members to sue a doctor who performs an abortion after cardiac activity is detected in an embryo.

However, he added he had concerns about whether the law was constitutional.

Writing in a letter to Lieutenant Governor Janice McGeachin, who is also president of the Idaho state senate, he said: “I stand in solidarity with all Idahoans who seek to protect the lives of pre-born babies.

“While I support the pro-life policy in this legislation, I fear the novel civil enforcement mechanism will in short order be proven both unconstitutional and unwise.”

It comes after new rules were introduced in Texas last year that meant women in the state are banned from terminating their pregnancy once a foetal heartbeat is detected, which can often occur as early as six weeks and many times before they even realise they’re pregnant.

The Texas statute authorises lawsuits against clinics, doctors and anyone who “aides or abets” an abortion that is not permitted by law.

The law in Idaho is scheduled to take effect 30 days after the signing, but court challenges are expected due to opponents calling it unconstitutional.

The law will allow the father, grandparents, siblings, aunts and uncles of a “pre-born child” to each sue an abortion provider for a minimum of $20,000 (£15,000) in damages within four years after the abortion.

The bill is ‘blatantly unconstitutional’

Rapists can’t file a lawsuit under the law, but a rapist’s relatives could.

Idaho Democrat Representative Lauren Necochea said: “The vigilante aspect of this bill is absurd.

“Its impacts are cruel, and it is blatantly unconstitutional.”

A Planned Parenthood official called the law unconstitutional and said the group was “committed to going to every length and exploring all our options to restore Idahoans’ right to abortion”.

“I want to emphasise to everyone in Idaho that our doors remain open. We remain committed to helping our patients access the health care they need, including abortion,” said Rebecca Gibron of Planned Parenthood Great Northwest, Hawaii, Alaska, Indiana and Kentucky, which operates Idaho’s three abortion clinics.

But those who back the law said it is Idaho’s best opportunity to severely restrict abortions in the state after years of trying.

Most recently, the state last year passed a six-week abortion ban law, but it required a favourable federal court ruling in a similar case to take effect, and that hasn’t happened.

A number of other states are pursuing similar laws, including Tennessee, which introduced a Texas-styled abortion bill earlier this week.