A lesbian couple form the US state of South Dakota plan to challenge the state’s same-sex marriage ban by marrying out-of-state.

Nancy Robrahn, 68, and Jennie Rosenkranz, 72, have been together for 27 years. They plan to marry in the state of Minnesota, before coming back to South Dakota to challenge its ban on equal marriage, and recognising same-sex marriages.

The couple’s journey to get married began on Monday at the Pennington County Courthouse, where they were refused a marriage licence form.

Their Minneapolis wedding, however, will go ahead, and is to be officiated by mayor Betsy Hodges.

The state banned same-sex marriage in 1996 through its Legislature, and then ten years later voters approved a constitutional amendment defining marriage as between one man and one woman.

The couple will be represented by Minneapolis lawyer Joshua Newville. He said two other couples are set to join the case, before also challenging a federal provision which allows some states to refuse to recognise out-of-state same-sex marriages.

“We are approaching the time when end of life decisions and plans need to be made,” Robrahn said in a statement issued by Newville.

“There are many federal protections that will become available to us through this Minnesota marriage. We hope to see the day when couples like us don’t have to travel out of South Dakota to marry.”

South Dakota Attorney General Marty Jackley has said he is required by law to defend state laws, and its constitution. He has reiterated that he will do so, should a lawsuit be filed.

Currently there are only four US states without a lawsuit challenging such restrictions – Alaska, Montana, North Dakota and South Dakota.