(Adds analyst, lawsuit in Missouri, details; edits)
By Keith Coffman and Daniel Wallis
DENVER, June 26 (Reuters) - A left-leaning Colorado county defiantly issued marriage licenses to gay couples on Thursday, emboldened by a landmark decision by a regional appeals court that found in favor of same-sex marriage in neighboring Utah.
The move appeared to set up a showdown between the elected clerk of Boulder County and Colorado's Republican attorney general, who warned that such nuptials would be invalid in the absence of a final judicial resolution.
A panel of the U.S. Court of Appeals for the 10th Circuit in Denver ruled on Wednesday that Utah cannot ban same-sex couples from marrying, pushing the issue of gay marriage a step nearer the U.S. Supreme Court.
The appeals court, whose decisions apply to six states including Colorado, put its ruling on hold anticipating an appeal. Yet within hours of the decision, the Boulder County clerk's office began issuing marriage licenses to gay couples.
County Clerk Hillary Hall said she acted immediately because gay couples across the state had waited a long time to have civil authorities recognize their right to marry, and said she would not stop unless ordered by a court.
"Given the 10th Circuit's recent decision and the numerous other cases on this issue, I would be surprised if a judge in Colorado were willing to invalidate a marriage license simply because the parties to the marriage were the same sex," she said in a statement.
A clerk's office spokeswoman said two licenses were issued on Wednesday, a further 15 by midday Thursday, and that more would likely be given out during the day.
No other clerk's office within the jurisdiction of the 10th Circuit has announced plans to follow Boulder's move, although a clerk in Missouri issued four licenses on Wednesday in violation of a state ban, prompting a lawsuit from the state attorney general.
Colorado Attorney General John Suthers said a state constitutional bar on same-sex marriages remained in force.
"(The) decision by the 10th Circuit Court of Appeals was stayed by the Court and has not gone into effect even in Utah, let alone in Colorado," he said. "Any marriage licenses issued to same-sex couples in Colorado before a final court resolution of the issue are invalid."
Boulder, home to the University of Colorado at the foothills of the Rocky Mountains, has acted in favor of gay marriage before. In 1975, its county clerk issued marriage licenses to several gay couples before being ordered to stop by the then-attorney general, who ruled such unions were illegal.
Wednesday's 10th Circuit ruling followed a series of decisions by federal district judges across the nation striking down state gay marriage bans as unconstitutional in rulings that could substantially expand U.S. gay marriage rights if upheld.
The 2-1 decision also marked the first time a regional appeals court has ruled on gay marriage since the Supreme Court made the federal government extend benefits to legally married same-sex couples a year ago.
At issue now in Boulder is whether the stay of the 10th Circuit's ruling applies to all the region's states, or only Utah.
Should the state attorney general take action to halt the marriages, he could be in for a tough fight, according to Jennifer Hendricks, a family and constitutional law expert at the University of Colorado.
She said it was hard to know if a judge could be found who would rule against a decision of the 10th Circuit that is likely headed to the Supreme Court, even with that decision stayed.
"There'd certainly be lots of questions about Colorado state law in terms of who has authority to do things like issue marriage licenses," she said. (Reporting by Keith Coffman and Daniel Wallis; Editing by Cynthia Johnston, Jim Loney and Eric Beech)