Voters in the US state of Minnesota will decide next year whether to place a constitutional ban on gay marriage.

Late on Saturday night, the state House voted to allow voters to decide the issue.

Minnesota already bans gay marriage but opponents argue that a constitutional amendment is needed to strengthen the ban and ward off challenges from gay rights campaigners.

In the 18-month run up to the vote, both sides are expected to spend millions of dollars on persuading voters to agree with them.

The House voted 70-62 for the referendum. Four Republican voted against it. One, John Kriesel, explained why he did so.

Mr Kriesel, who lost both legs when he was injured by an improvised explosive device when serving in Iraq, told lawmakers: “This amendment doesn’t represent what I went to fight for.”

To the sound of gay marriage campaigners chanting outside, he said: “Hear that out there? That is what I went to fight for and I am glad about that.”

Last week, a poll suggested that just over half of Minnesota voters do not want a constitutional ban on same-sex marriage.

The Star Tribune Minnesota poll found that 55 per cent of respondents did not want a ban, compared to 39 per cent who said they did.

The newspaper said this was a “sharp reversal” of poll results taken seven years ago.

Gay rights campaigners have already set up an umbrella group called Minnesotans United for All Families to campaign against a ban.