A senior US bishop has called on all Roman Catholics to oppose gay marriage by supporting a marriage amendment law.

Bishop William Skystad, president of the US Conference of Catholic Bishops said a law protecting marriage was the only way to ban gay unions in the constitution.

He wrote in an open letter, “There is a growing sense shared by many people, including a wide range of religious leaders, that a marriage protection amendment is the only federal-level action that ultimately will protect and preserve the institution of marriage.”

The minister expressed hope that the Protection of Marriage Amendment would be introduced in the Senate in June, “In a matter of months we will have the opportunity once again to stand publicly in support of marriage as the God-given union of a man and a woman.

“I am aware that the time is short for taking action, so I urge you to do whatever you can, given the situation and the resources available to you.”

Jeremy Leaming, spokesman for Americans United for the Separation of Church and State, condemned Mr Skylstad’s plans, he said: “I think this confirms what our organisation has been arguing for a long time, that the religious majority in America wants the federal marriage amendment to enshrine their idea of marriage in the Constitution and have it imposed on everyone else, and other religions are rendered second class in essence.

“We have a constitution that mandates a separation of church and state, and we have these people trying to enshrine their religious views in that very document.”

A gay group criticised the bishop for spending so much time on gay issues and called for love and inclusion in time for Easter, Sam Sinnett president of Dignity USA, said: “With all the problems in the world, our bishops are putting their time and moral authority into this wedge issue in order to promote a conservative political issue rather than a Christian one.

“We would call upon all the bishops to pursue the Christian values of love and inclusion rather to demonise gay and lesbian people who are seeking out civil rights.”