AUSTIN, Texas -- A federal judge declared a same-sex marriage ban in deeply conservative Texas unconstitutional Wednesday, the latest in a series of victories for gay rights activists who are challenging bans in dozens of states around the U.S.

The Texas ruling will allow the second-most populous U.S. state to continue enforcing the ban pending an appeal that will likely go to the U.S. Supreme Court, where a tangled web of lawsuits is expected to end up next year. It follows similar recent decisions in Utah, Oklahoma and Virginia, as proponents of same-sex marriage have been emboldened by last year's landmark Supreme Court ruling that said legally married gay couples could not be denied federal benefits.

Judge Orlando Garcia issued a preliminary injunction Wednesday after two gay couples challenged a Texas constitutional amendment and a longstanding law.

"Without a rational relation to a legitimate governmental purpose, state-imposed inequality can find no refuge in our United States Constitution," Garcia wrote. "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."

Garcia said the couples are likely to win their case and the ban should be lifted, but said he would not enforce his ruling pending one by a court of appeals, which already is hearing two other states' cases. He also will give Texas time to appeal to a federal appeals court.

Garcia, appointed by President Bill Clinton, is the first judge in his conservative New Orleans-based court to reach such a decision. Texas Attorney General Greg Abbott, who also is the leading Republican candidate to succeed Gov. Rick Perry, promised to appeal the decision.

Todd Staples, a candidate for lieutenant governor who drafted the Texas constitutional amendment banning gay marriage, 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.

At least 17 states, mostly in the Northeast, and the Washington capital district now allow marriage of same-sex couples.

The Supreme Court, in its decision last June, declined to rule on the merits of a voter-approved California law that defined marriage as between a man and a woman. The effect of the decision was to allow same-sex unions to resume in California, but the high court said nothing about the right to marry. It did order the federal government to recognize valid same-sex marriages by striking down the part of the Clinton-era Defence of Marriage Act that denied same-sex married couples the federal benefits and rights enjoyed by heterosexual couples.

Every judge who has decided a same-sex marriage case since last year's Supreme Court ruling has come down on the side of gay marriage and has drawn heavily on the high court's opinions.

That has left opponents of gay marriage scrambling, and some have called for extraordinary measures in response.

A bill recently introduced in Congress by conservative Republicans titled the State Marriage Defence Act would require the federal government to respect each state's determinations of its residents' marital status when applying federal law. It has no chance of passage in the Democratic-led Senate.

In Missouri, where voters approved a gay-marriage ban in 2004, Republican legislators filed articles of impeachment against Democratic Gov. Jay Nixon after he ordered his administration to accept joint tax returns from same-sex couples who were legally married in other states.

Bills are making their way through several state legislatures, some intended to protect gay-marriage bans, others to protect individuals or businesses who, for religious reasons, don't want to serve same-sex couples.

The first such bill to be passed by a state legislature is Arizona's, which is being weighed by Gov. Jan Brewer. Business groups, gay rights supporters and even fellow Republicans urged her to use her veto power, and there is widespread speculation that she will, but she has not said how she'll act.

The legislation allows any business, church or person to cite the law as a defence in any action brought by the government or individual claiming discrimination.

Proponents frequently cite the case of a photographer in neighbouring New Mexico who was sued after refusing to take wedding pictures of a gay couple and say Arizona needs a law to protect people from heavy-handed actions by federal courts.