LONDON — Lawmakers in David Cameron’s Conservative Party, who last week voted against him over his policy toward the European Union (EU), were Monday on course to oppose him again, over plans to introduce gay marriage.

On May 15, 116 Conservatives made history by voting to express “regret” that there was no mention of a referendum on EU membership in the government’s legislative program outlined this month. That was only defeated thanks to the support of Liberal Democrat and opposition Labour Party lawmakers.

The Times and Telegraph newspapers reported that afterward a senior Conservative described Tory activists as “mad swivel-eyed loons.” Andrew Feldman, the party’s co-chairman, issued a statement denying it was him.

The gay-marriage bill returns to the House of Commons for debate Monday. On its last appearance on Feb. 5, 136 Tories voted against, while 127 voted in favor. It too was passed with help from the Liberal Democrats, the junior partners in the ruling coalition, and Labour. Opponents have put down amendments today that the government said could wreck the legislation.

Nick Herbert, a Conservative lawmaker and supporter of the bill, told the BBC it was causing “unease” among older party members.

“Absolutely none of us either want to upset or lose activists, and certainly what I would like to do is to reassure them that this is a bill that does no harm at all,” he said.

One of the amendments tabled for debate involves extending civil partnerships to heterosexual couples, which the government said could delay the bill by two years as the public is consulted further.

Culture Secretary Maria Miller said the amendment “would throw up significant challenges,” calling it a “complication that is not necessary at this point in time.”

Activists say Cameron’s backing for gay marriage is driving Conservative voters to support the U.K. Independence Party, which made gains in local elections this month at the expense of the ruling parties.