One Washington state voter-initiative to ban equal marriage has failed to gain enough support.

Backers of Initiative 1192, which sought to restrict marriage to heterosexual couples, said they had fallen “woefully short” of the number of signatures needed to get their measure on the ballot.

However, a competing measure, Referendum 74, has already won enough support to appear on the November ballot, meaning Washington voters will decide whether gay couples should have the right to marry.

The campaign group behind Initiative 1192, Protect Marriage Washington, said they were around 140,000 signatures short of the 240,000 needed to get on the ballot.

Group leader Stephen Pidgeon told Reuters: “We are woefully short, not just a little bit short.”

He added: “The truth is there was a competing measure … and the referendum, let’s just say they were very hot in their competition and even though we supported their effort they did not support ours.”

Washington state legalised gay marriage earlier this year but the legislation is on hold.

Every state which has had a referendum on equal marriage so far has moved to reject it, including, most notoriously, California through the Proposition 8.

But according to a left-leaning think-tank, Public Policy Polling, voters in Washington are set to approve equal marriage.

Nearly 51 per cent supported equal marriage, with 42 per cent opposed, while the rest remain undecided. The error margins are roughly plus or minus 3%. Among democrats in the state, the support for equal marriage jumped to 82%.

Recent polls have found that more Americans support equal marriage than oppose it, a reversal of the historic trend that first began to be observed a year and a half ago.