A US federal judge declared on Wednesday that a ban on same-sex marriage in the southern state of Texas is unconstitutional.
The ruling follows similar decisions recently made in Utah, Oklahoma and Virginia.
LGBT rights campaigners in these states have been emboldened by a landmark Supreme Court ruling last year that said legally married homosexual couples could not be denied the federal benefits enjoyed by married heterosexual couples.
However, the state will continue to enforce the ban pending an appeal from the state of Texas that will likely go to the US Supreme Court, where a number of lawsuits from different states are expected to be heard next year.
Judge Orlando Garcia issued the preliminary injunction on Wednesday after two gay couples challenged a Texas constitutional amendment and a longstanding law.
Garcia said the couples are likely to win their cases and the ban should be lifted.
He is the first judge in his conservative New Orleans-based court to reach such a decision.
He wrote on Wednesday:"These Texas laws deny plaintiffs access to the institution of marriage and its numerous rights, privileges, and responsibilities for the sole reason that Plaintiffs wish to be married to a person of the same sex.
"Without a rational relation to a legitimate governmental purpose, state-imposed inequality can find no refuge in our United States Constitution," Garcia added.
Texas Attorney General Greg Abbott, who also is the leading Republican candidate to succeed Governor Rick Perry, said he would appeal the ruling.
Todd Staples, a candidate for lieutenant governor who drafted the Texas constitutional amendment banning gay marriage, also denounced the court's decision.
"I am disappointed that judicial activism is once again trying to trump the will of the people. This ruling is the poster child of the culture war occurring in America today," he said.
Meanwhile, at least 17 states, mostly in the Northeast, and the Washington capital district, now allow marriage of same-sex couples.
In its decision last June, the Supreme Court allowed same-sex unions to resume in California, but did not rule on the right to marry.
It instead ordered the federal government to recognise same-sex marriages as valid by striking down the part of the Clinton-era Defense of Marriage Act that denied same-sex married couples federal benefits.
Since the high court ruling, every judge deciding on a same-sex marriage case has come down in favour of gay marriage.
However, conservatives against same-sex marriage have not been persuaded by the high court precedent.
A bill recently introduced in Congress by conservative Republicans titled the State Marriage Defense Act would require the federal government to respect each state's determinations of its residents' marital status when applying federal law.
But it is unlikely to pass in the Democratic-led Senate.