A federal judge has ruled that Oklahoma’s voter-approved ban on gay marriage is unconstitutional, raising the prospect of a widening battle against similar provisions in other conservative states.

Terence Kern, a senior district judge, ruled that Oklahoma's ban on same-sex marriages or civil unions was a violation of the right to equal protection and due process under the US constitution. It puts the state on the same legal track as Utah, where a federal judge ruled against the voter-approved constitutional ban last month.

The Oklahoma ruling does not open the doors immediately to gay marriages, as Kern imposed a stay on the issue pending appeal by state authorities. In Utah, there was a flurry of same-sex weddings before the US supreme court stepped in and imposed a stay while the appeals process played out.

Oklahoma passed its ban in 2004 following a referendum of voters in which an overwhelming 76% concurred with the statement that “marriage in this state shall consist only of the union of one man and one woman”.

It becomes the latest state to feel the impact of the US supreme court's landmark judgment last June that struck down the Defense of Marriage Act (Doma) – a federal law that prohibited the US government from recognising same-sex marriages in those states that have legalised them. Though the high court found that those people living in gay marriages had to be treated equally in terms of federal benefits, it did not touch the central question of whether gay marriage itself was constitutional – leaving the legal dispute to play out at state level in the lower courts.

That dispute is now sweeping at dramatic speed out of more liberal states on the coasts where gay marriage is increasingly the norm, into more conservative states in the heartlands. Recent cases have seen the courts overturn gay marriage bans in New Mexico and Utah.

The issue is now likely to wend its way back up the legal system until it reaches the US supreme court once again for an ultimate decision. In Utah, however, 1,300 same-sex couples got married during the 17 days in which marriage was legalised, before a stay was imposed by the supreme court on 6 January.

Last Friday, the Obama administration waded into the fray when the US justice department said it would recognise under federal law all those 1,300 marriages despite the enduring opposition of the state of Utah and the legal stay in place.

The addition of Oklahoma to the growing list of conservative states now in legal limbo over their gay marriage bans was greeted with glee by gay rights advocates. Chad Griffin, president of the Human Rights Campaign, said “Equality is not just for the coasts anymore. Today’s news from Oklahoma shows that time has come for fairness and dignity to reach every American in all 50 states.”