Justice Anthony Kennedy’s decision to retire has been met with fear by those who fear it could a reversal of US abortion laws.

The highly respected judge was a key defender of Roe v Wade during his nearly 30 years on the Supreme Court's bench, serving as a tie-breaking vote on challenges to the 1973 decision that made abortion legal in America.

His retirement means President Donald Trump will be able to nominate another justice the Court – bringing the balance to four justices in favour of Roe v Wade and five against it, many experts have concluded.

The news triggered massive outcry from women’s rights advocates and organisations, who warned the reversal of reproductive freedoms was imminent.

“With Trump nominating the next Supreme Court justice, our constitutional right to access legal abortion is in DIRE IMMEDIATE danger – along with the fundamental rights of all Americans,” tweeted NARAL, a pro-choice political advocacy group.

Legal analyst Jeffrey Toobin also warned on CNN: “You’re going to see 20 states pass laws banning abortion outright."

Erwin Chemerinsky, a constitutional scholar and the dean of Berkeley Law, agreed with his sentiment.

The attorney said it was “very likely” that Roe v Wade would be overturned under a new Court configuration, leaving it up to the states to determine their own abortions laws.

Donald Trump on Justice Kennedy: 'He will be missed, but he will be retiring'

About half of the states would choose to outlaw abortion outright, he estimated.

Fearing just such an outcome, activists urged women to call their representatives and demand that any of Mr Trump’s Supreme Court selections be shot down.

Others started fundraisers for abortion providers and advocacy groups within hours of Mr Kennedy's announcement. Some women even suggested starting up underground networks to help women in states where abortion was made illegal.

Supreme Court strikes down abortion restrictions Show all 23 1 /23 Supreme Court strikes down abortion restrictions Supreme Court strikes down abortion restrictions Abortion rights activists hold placards outside of the US Supreme Court ahead of an expected ruling on abortion clinic restrictions on June 27, 2016 in Washington, DC. Mandel Ngan/AFP/Getty Supreme Court strikes down abortion restrictions An abortion rights activist holds placards outside of the US Supreme Court before the Court struck down a Texas law placing restrictions on abortion clinics on June 27, 2016 in Washington, DC. The US Supreme Court on Monday struck down a Texas law placing a raft of restrictions on abortion clinics, handing a major victory to the "pro-choice" camp in the country's most important ruling on the divisive issue in a generation. Mandel Ngan/AFP/Getty Supreme Court strikes down abortion restrictions WASHINGTON, DC - JUNE 27: Pro-life activists pray on the steps of the United States Supreme Court on June 27, 2016 in Washington, DC. In a 5-3 decision, the U.S. Supreme Court struck down one of the nation's toughest restrictions on abortion, a Texas law that women's groups said would have forced more than three-quarters of the state's clinics to close. Pete Marovich/Getty Supreme Court strikes down abortion restrictions WASHINGTON, DC - JUNE 27: Texas abortion provider Amy Hagstrom-Miller looks on as Nancy Northup, President of The Center for Reproductive Rights speaks to the media outside of the U.S. Supreme Court on June 27, 2016 in Washington, DC. In a 5-3 decision, the U.S. Supreme Court struck down one of the nation's toughest restrictions on abortion, a Texas law that women's groups said would have forced more than three-quarters of the state's clinics to close. Pete Marovich/Getty Supreme Court strikes down abortion restrictions An abortion rights activist holds placards outside of the US Supreme Court before the Court struck down a Texas law placing restrictions on abortion clinics on June 27, 2016 in Washington, DC. The US Supreme Court on Monday struck down a Texas law placing a raft of restrictions on abortion clinics, handing a major victory to the "pro-choice" camp in the country's most important ruling on the divisive issue in a generation. Getty Supreme Court strikes down abortion restrictions Texas abortion provider Amy Hagstrom-Miller wipes a tear as she walks down the steps of the United States Supreme Court with Nancy Northup, President of The Center for Reproductive Rights on June 27, 2016 in Washington, DC. In a 5-3 decision, the U.S. Supreme Court struck down one of the nation's toughest restrictions on abortion, a Texas law that women's groups said would have forced more than three-quarters of the state's clinics to close. Pete Marovich/Getty Supreme Court strikes down abortion restrictions Abortion rights activists cheer after the US Supreme Court struck down a Texas law placing restrictions on abortion clinics, outside of the Supreme Court on June 27, 2016 in Washington, DC. In a case with far-reaching implications for millions of women across the United States, the court ruled 5-3 to strike down measures which activists say have forced more than half of Texas's abortion clinics to close. Mandel Ngan/AFP/Getty Supreme Court strikes down abortion restrictions Abortion rights activists cheer after the US Supreme Court struck down a Texas law placing restrictions on abortion clinics, outside of the Supreme Court on June 27, 2016 in Washington, DC. In a case with far-reaching implications for millions of women across the United States, the court ruled 5-3 to strike down measures which activists say have forced more than half of Texas's abortion clinics to close. Mandel Ngan/AFP/Getty Supreme Court strikes down abortion restrictions WASHINGTON, DC - JUNE 27: Abortion rights activists Morgan Hopkins of Boston, left, and Alison Turkos of New York City, celebrate on the steps of the United States Supreme Court on June 27, 2016 in Washington, DC. In a 5-3 decision, the U.S. Supreme Court struck down one of the nation's toughest restrictions on abortion, a Texas law that women's groups said would have forced more than three-quarters of the state's clinics to close. Pete Marovich/Getty Supreme Court strikes down abortion restrictions Abortion rights activists cheer after the US Supreme Court struck down a Texas law placing restrictions on abortion clinics, outside of the Supreme Court on June 27, 2016 in Washington, DC. In a case with far-reaching implications for millions of women across the United States, the court ruled 5-3 to strike down measures which activists say have forced more than half of Texas's abortion clinics to close. Getty Supreme Court strikes down abortion restrictions WASHINGTON, DC - JUNE 27: Abortion rights activists Morgan Hopkins of Boston, left, and Alison Turkos of New York City, celebrate on the steps of the United States Supreme Court on June 27, 2016 in Washington, DC. In a 5-3 decision, the U.S. Supreme Court struck down one of the nation's toughest restrictions on abortion, a Texas law that women's groups said would have forced more than three-quarters of the state's clinics to close. (Photo by Pete Marovich/Getty Images) Getty Images Supreme Court strikes down abortion restrictions Abortion rights activist Morgan Hopkins of Boston, celebrates on the steps of the United States Supreme Court on June 27, 2016 in Washington, DC. In a 5-3 decision, the U.S. Supreme Court struck down one of the nation's toughest restrictions on abortion, a Texas law that women's groups said would have forced more than three-quarters of the state's clinics to close. Pete Marovich/Getty Supreme Court strikes down abortion restrictions Abortion rights activists embrace after the US Supreme Court struck down a Texas law placing restrictions on abortion clinics, outside of the Supreme Court on June 27, 2016 in Washington, DC. In a case with far-reaching implications for millions of women across the United States, the court ruled 5-3 to strike down measures which activists say have forced more than half of Texas's abortion clinics to close. AFP/Getty Images Supreme Court strikes down abortion restrictions Abortion rights activists hold placards outside of the US Supreme Court ahead of a ruling on abortion clinic restrictions on June 27, 2016 in Washington, DC. In a case with far-reaching implications for millions of women across the United States, the court ruled 5-3 to strike down measures which activists say have forced more than half of Texas's abortion clinics to close. Mandel Ngan/AFP/Getty Supreme Court strikes down abortion restrictions In a case with far-reaching implications for millions of women across the United States, the court ruled 5-3 to strike down measures which activists say have forced more than half of Texas's abortion clinics to close. Mandel Ngan/AFP/Getty Supreme Court strikes down abortion restrictions Abortion rights activists cheer after the US Supreme Court struck down a Texas law placing restrictions on abortion clinics, outside of the Supreme Court on June 27, 2016 in Washington, DC. In a case with far-reaching implications for millions of women across the United States, the court ruled 5-3 to strike down measures which activists say have forced more than half of Texas's abortion clinics to close. AFP/Getty Images Supreme Court strikes down abortion restrictions Abortion rights activists cheer after the US Supreme Court struck down a Texas law placing restrictions on abortion clinics, outside of the Supreme Court on June 27, 2016 in Washington, DC. In a case with far-reaching implications for millions of women across the United States, the court ruled 5-3 to strike down measures which activists say have forced more than half of Texas's abortion clinics to close. / AFP / MANDEL NGAN (Photo credit should read MANDEL NGAN/AFP/Getty Images) AFP/Getty Images Supreme Court strikes down abortion restrictions Abortion rights activists cheer after the US Supreme Court struck down a Texas law placing restrictions on abortion clinics, outside of the Supreme Court on June 27, 2016 in Washington, DC. In a case with far-reaching implications for millions of women across the United States, the court ruled 5-3 to strike down measures which activists say have forced more than half of Texas's abortion clinics to close. / AFP / MANDEL NGAN (Photo credit should read MANDEL NGAN/AFP/Getty Images) AFP/Getty Images Supreme Court strikes down abortion restrictions Abortion rights activists embrace after the US Supreme Court struck down a Texas law placing restrictions on abortion clinics, outside of the Supreme Court on June 27, 2016 in Washington, DC. In a case with far-reaching implications for millions of women across the United States, the court ruled 5-3 to strike down measures which activists say have forced more than half of Texas's abortion clinics to close. AFP/Getty Images Supreme Court strikes down abortion restrictions Pro-choice activist, Alissa Manzoeillo, of Washington, D.C. waits for rulings in front of the U.S. Supreme Court on June 27, 2016 in Washington, DC. A ruling is expected in Whole Woman's Health v. Hellerstedt, a Texas case the places restrictions on abortion clinics, as well as rulings in the former Virginia Governor's corruption case and a gun rights case. Pete Marovich/Getty Supreme Court strikes down abortion restrictions Abortion rights activists cheer after the US Supreme Court struck down a Texas law placing restrictions on abortion clinics, outside of the Supreme Court on June 27, 2016 in Washington, DC. In a case with far-reaching implications for millions of women across the United States, the court ruled 5-3 to strike down measures which activists say have forced more than half of Texas's abortion clinics to close. Supreme Court strikes down abortion restrictions Abortion rights activists hold placards outside of the US Supreme Court ahead of an expected ruling on abortion clinic restrictions on June 27, 2016 in Washington, DC. AFP/Getty Images Supreme Court strikes down abortion restrictions Abortion rights activists hold placards outside of the US Supreme Court ahead of an expected ruling on abortion clinic restrictions on June 27, 2016 in Washington, DC. AFP/Getty Images

But Megan Donovan, a senior policy manager at research and advocacy group Guttmacher Institute, said the rollback of abortion rights may be slower – and less obvious – than many anticipated.

She said Americans should expect to see the protections of Roe v Wade “drastically undermined” before they were overturned.

“Rather than the court stripping constitutional protections for abortions, we could see [the Court] really open the door for an ever-increasing array of restrictions that have the effect of making it harder and harder – and in some places impossible – for people to access abortion,” she added.

Although the Supreme Court – with Mr Kennedy's help – shot down some of the most stringent restrictions in 2016, many states already have laws on the books limiting women's access to abortion.

Forty-three states already prohibit abortions after a certain point in a pregnancy, and 27 require women to wait a specific amount of time before receiving one, according to the Guttmacher Institute. More recently, pro-life legislators have pushed for bans on specific types of abortions, such as those completed by taking a pill at home.

With a sympathetic court, Ms Donovan said, some state legislators may feel empowered to implement even more of these anti-choice restrictions.