A judge has struck down a ban on same-sex marriage in Arkansas, saying the US state has “no rational reason” for preventing gay couples from marrying.

Pulaski County Circuit Judge Chris Piazza ruled that the 2004 voter-approved amendment to the state constitution violates the rights of same-sex couples.

“This is an unconstitutional attempt to narrow the definition of equality,” Piazza wrote. “The exclusion of a minority for no rational reason is a dangerous precedent.”

The ruling came nearly a week after state Attorney-General Dustin McDaniel announced he personally supports gay marriage rights but that he will continue to defend the constitutional ban in court. McDaniel’s office said he would appeal the ruling.

A lawyer for the group of same-sex couples challenging the prohibition said he hadn’t talked with his clients and didn’t know if they would seek marriage licences when county offices open on Monday morning.

“If I was them, I would be there waiting for the door to open,” lawyer Jack Wagoner said.

The amendment banning gay marriage was passed with the overwhelming support of Arkansas voters. Piazza’s ruling also overturns a 1997 state law banning gay marriage.

In his ruling, Piazza cited the US Supreme Court’s 1967 decision that invalidated laws on interracial marriage.

The US Supreme Court last year ruled that a law forbidding the federal government from recognising same-sex marriages was unconstitutional. Since then, lower-court judges have repeatedly cited the decision when striking down some of the same-sex marriage bans that were enacted after Massachusetts started recognising gay marriages in 2004.

Federal judges have ruled against marriage bans in Michigan, Oklahoma, Utah, Virginia and Texas, and ordered Kentucky, Ohio and Tennessee to recognise same-sex marriages from other states.