A group of Christian pastors has vowed to launch a civil disobedience campaign if the US Supreme Court decides to allow same-sex marriage in a ruling expected this month.

Dr. Rick Scarborough, a Baptist pastor and president of the activist group Vision America, said if it ever came to deciding whether to follow God's law or law passed by politicians, he would follow God's law. He said he and other Christian evangelicals will refuse to recognise any Supreme Court ruling that favours same-sex marriage.

"I will continue to preach that homosexuality is a sinful lifestyle," Scarborough told News Radio 1200 WOAI.

He said about 40,000 pastors and church leaders have signed a petition at www.defendmarriage.org to "resist all government efforts to require them to accept gay marriage, and they will accept any fine and jail time to protect their religious freedom and the freedom of others."

"Marriage as existing solely between one man and one woman precedes civil government," according to the petition.

Scarborough said "never before has a civilisation in the history of man approved of marriage between two men and two women."

He said people who refuse to accept an order making same-sex marriage acceptable face difficulties including companies and religious organizations that may face repercussions if they do not hire gay employees.

Schools may be forced to teach that gay marriage is acceptable lest they lose their tax exempt status, he said, adding that private businesses like bakeries and florists have been sued for refusing to provide service to same-same weddings.

Scarborough defended their planned civil disobedience campaign saying that it has a long history in the US, including the fight against segregation by Dr. Martin Luther King.

"Dr. King, in the Birmingham Jail, said Christians have a 'duty to obey and respect God's law," Scarborough said.

In their petition, the signatories said, "On the matter of marriage, we stand in solidarity. We affirm that marriage and family have been inscribed by the Divine Architect into the order of Creation. Marriage is ontologically between one man and one woman, ordered toward the union of the spouses, open to children and formative of family."

It said the government should defend marriage and not undermine it.

"No civil institution, including the United States Supreme Court or any court, has authority to redefine marriage," according to the petition.