Adultery is no longer illegal in South Korea, after the country's Constitutional Court struck down a more than 60-year-old law.

Violators of the law could previously be imprisoned for up to two years, and its validity had already been unsuccessfully challenged in the court four times.

On this occasion, seven of the nine justices on the bench determined the 1953 statute was unconstitutional.

"Even if adultery should be condemned as immoral, state power should not intervene in individuals' private lives," presiding justice Park Han-Chul said.

"Public conceptions of individuals' rights in their sexual lives have undergone changes."

Prosecutions under the law have been falling in recent years, with almost 5,500 people formerly arraigned on adultery charges in the past six years.

Meanwhile, prison terms have become rare, with 216 people jailed in 2004, but only 22 imprisoned since 2008.

The downward trend was partly a reflection of changing societal trends in a country where rapid modernisation has frequently clashed with traditionally conservative norms.

Under the law, adultery could only be prosecuted if an injured party complained, and any case was closed immediately if the plaintiff dropped the charge - a common occurrence that often involved a financial settlement.

The law was grounded in a belief that adultery challenged the social order and damaged families, but critics insisted it was outdated and represented state overreach into people's private lives.

The debate over its future had simmered away for years, bubbling over from time to time especially if a public figure fell foul of the statute.

Such was the case in 2008 when one of the country's best-known actresses, Ok So-ri, was given an eight-month suspended sentence for having an adulterous affair.

At that time, Ok unsuccessfully petitioned the Constitutional Court, arguing that the law amounted to a violation of her human rights in the name of revenge.

Ok's 2008 petition came close to success, with five of the required six judges deeming the statute unconstitutional.

Law originally designed to protect women's rights

The law was introduced primarily to protect the rights of women in marriages, but made South Korea one of the few non-Muslim countries to consider marital infidelity a criminal act.

But even socially conservative civic groups who had supported the legislation in the past acknowledged that times had changed.

"Adultery must be censured morally and socially, but such a law is inappropriate in a modern society," activist with the group Healthy Families, Ko Seon-Ju, said.

"It used to be an effective legal tool to protect female rights, but equal rights legislation has improved.

"Adultery is an issue that should be dealt with through dialogue between the partners, not by law."

While the adultery law may have been ruled out of existence, social disapproval of marital infidelity remains potent.

In April last year, South Korea blocked the newly launched Korean version of the global adultery hook-up site Ashley Madison, saying it threatened family values.

AFP