David Burrows, a Conservative Party member leading the opposition to same-sex marriage bill, said allowing tax breaks for married couples would have helped "soften the blow" of gay marriages, The Daily Telegraph reported.
The tax breaks were supposed to have been introduced by the House of Commons, but government officials said Friday the policy would not be featured in March's budgets.
About 120 conservative members of Parliament are expected to vote against the proposal to legalize same-sex marriage Tuesday, the newspaper said.
"Colleagues will wonder why we are spending time and energy redefining marriage when we have no mandate to do so and at the same time not pushing through our manifesto commitment to support marriage in the tax system," said Burrows. "This will affect the mood."
Prime Minister David Cameron's office said he plans to try to persuade Conservative members to support the bill ahead of Tuesday's vote.
"It is a free vote but clearly the prime minister is in favor of the legislation," a spokesman said. "He firmly believes that it is the right thing to do and of course he will encourage others to vote for it."