The Conservative party is split over the Government's gay marriage plans, the oldest Tory think tank has said.

The Government plans to reform marriage so that the definition is no longer restricted to a union between a man and a woman, but can also include same-sex couples.

Ben Harris, chairman of the Bow Group, said that the adoption of legislation for gay marriage in addition to civil partnerships would "create an illogical and opaque structure that offers only homosexuals the option of civil union or gay marriage, both before the state with no discernible difference between the legal or religious status of either".

"What we are seeing here is genuine fatigue among a party rank and file, disappointed both at the suggested policy itself, and the reasons behind it," he said.

"The UK already has a system of 'civil partnership' for homosexuals which carries the same legal status as marriage before the state."

He continued: "The concern within the Conservative party is that this is a poorly thought out policy designed to cynically win support in the short term in the homosexual community, without due care and consideration to the existing institution of marriage in Britain."

A recent poll conducted by ComRes found that only 41% of Conservative MPs support David Cameron's plans to introduce gay marriage.

Mr Harris said it should be a "greater concern" to the Prime Minister that, in addition to strong opposition within his own party, recent polls by ComRes and Sky News have revealed that 70% of people believe marriage should remain a "lifelong exclusive commitment between a man and a woman".

Tory MP David Burrowes, who has received death threats over his opposition to the plans, said: "The strength of opposition in some quarters appeared to have taken UK government ministers by surprise.

"I think this fairly reflects the fact that there is a significant number of MPs, particularly Conservative MPs, who are concerned about whether we should legislate on marriage."