Lord Carey says the plan needs greater scrutiny
Plans for civil partnership ceremonies in churches could result in some clergy being prosecuted, a former Archbishop of Canterbury has warned.
In a letter to the Times newspaper, Lord Carey suggests ministers could face claims of discrimination if they fail to conduct the ceremonies.
Legislation for England and Wales, which also covers other places of worship, is part of the Equality Bill.
Its supporters say ministers would not have to act against their conscience.
Lord Carey's letter was prompted by Labour peer Lord Alli's amendment to the bill to allow civil partnerships on religious premises.
The legislation had its third reading in the House of Lords on Tuesday and it will now go back to the Commons for final approval.
In his letter, Lord Carey says: "How long will it be before church ministers are threatened with legal proceedings if they perform marriages between a man and a woman, but not civil partnerships?"
He says the amendment risked blurring the "clear distinction" between homosexual partnerships and heterosexual marriage.
Lord Carey says the plans were being "tacked on" to an existing bill "without a proper chance for scrutiny".
John McManus, Religious Affairs producer, BBC News
Lord Carey's criticism, and yesterday's delivery of a petition to Harriet Harman, are the latest skirmishes between faith groups who feel that their beliefs and privileges are being undermined by a government that wants to bring previously marginalised groups into the mainstream.
In recent years Christians, Muslims and Jews have all taken on the government over issues ranging from sex education in schools to adoption of children by gay couples and attempts to relax the rules on assisted suicide.
The issues can become even more complicated because most of these faith groups are "broad churches", with a variety of opinion on different topics.
Nevertheless, Lord Carey's letter will have encouraged more traditional believers of all religions that their fight to preserve marriage as uniquely involving a man and a woman is one that is still worth having.
"If there is a genuine need for a change in the law this should be done properly by revisiting the Civil Partnerships Act," he writes.
Lord Carey's intervention is not the first time he has spoken on the subject of civil partnerships.
This time, he is being supported by groups such as Christian Concern for our Nation, which on Tuesday handed in a 6,000-signature petition to the Equalities Minister Harriet Harman.
It calls on the government to vote down the amendment because it threatens religious freedom.
Before the introduction of civil partnerships in 2005, Lord Carey told Christians not to be suspicious of the new legal entities, but insisted that they should not be the first step towards gay partnerships being classed as marriages.
Lord Alli has insisted that the proposals in his amendment are entirely voluntary, with ministers, rabbis or imams who object to homosexuality being able to legally refuse to conduct the ceremonies.
He is supported by other Church of England clergy, including the Bishop of Salisbury, and the leaders of Liberal Judaism and the Quakers, who say they want to help gay couples celebrate their unions.