The Massachusetts supreme court gave the state's lawmakers 180 days to work out how to ensure equality at the altar after ruling 4 to 3 that same-sex couples were legally entitled to get married under its constitution.

"Whether and whom to marry, how to express sexual intimacy, and whether and how to establish a family - these are among the most basic of every individual's liberty and due process rights," the majority opinion said. "And central to personal freedom and security is the assurance that the laws will apply equally to persons in similar situations."

The state's Republican governor, Mitt Romney, immediately criticised the ruling, saying: "Marriage is an institution between a man and a woman. I will support an amendment to the Massachusetts constitution that makes that expressly clear. Of course, we must provide basic civil rights and appropriate benefits to nontraditional couples, but marriage is a special institution that should be reserved for a man and a woman."

But Mary Bonauto, the lawyer who represented the seven gay couples who sued the state, said that the legislators would not be able to change the state constitution within the 180-day timeframe. She insisted that the only task assigned to the legislature was to come up with the legal changes to make gay marriage possible.

"This is a very good day for gay and lesbian families in Massachusetts and throughout the country," Ms Bonauto said.

The court, however, stopped short of allowing marriage licences to be issued to the couples who challenged the law until the legislature had come up with a means of enforcing the ruling.

The decision marks yet another victory for gay rights activists in a tumultuous year that has seen the ordination of the Episcopal church's first openly gay bishop and a United States supreme court judgment which declared anti-sodomy laws unconstitutional, effectively making consensual sex between gay couples legal.

But it is also likely to make the issue of gay rights the central social issue in next year's presidential elections, pitting the Christian right - the bedrock of Republican support - against gay rights campaigners and civil libertarians.

Gay rights groups are now braced for a backlash.

When the Hawaii supreme court ruled that the denial of gay marriages was unconstitutional in 1993, the legislature quickly intervened with legislation banning gay marriage. Dozens of other states moved pre-emptively to ward off a similar development, while Congress moved to deny federal recognition of homosexual marriage and allowed states to ignore same-sex unions licensed elsewhere.

Polls show American public opinion is in flux on the issue. Attitudes towards homosexuality have softened in recent years. In a USA Today poll in July nearly one third said they had become more accepting of gay people in recent years, compared to just 8% who had become less accepting. Almost 90% said gay people should have equal rights in terms of job opportunities.

However, while the nation may have become more tolerant it is divided when it comes to granting gay couples full equality. According to a poll released yesterday, most Americans, 55%, say they felt that homosexuality was a sin, while 33% did not. And while 32% favoured gay marriage, 59% opposed it.

The gulf between the court rulings and a rapidly shifting public opinion has provided the space for an intense cultural and political clash. The gay advocacy group the Human Rights Campaign has launched a $1m advertising campaign to put gay marriage in a positive light.