A Catholic Tory MP has voiced concern over the government’s plans to allow gay couples to marry, suggesting the law on consummation may be affected to the detriment of straight couples.

Currently, the Matrimonial Causes Act 1973 allows a straight couple’s marriage to be voided if one party was unable or unwilling to consummate the union.

According to case law, the consummation must be “ordinary and complete” rather than “partial and imperfect”, but there is no such requirement on civil partners.

Edward Leigh, the Conservative MP for Gainsborough, warned of the unwanted effect allowing gay couples to marry may have on religious straight couples if such a move changes the law of consummation.

Mr Leigh told The Sun: “It will have profound effects on the ability of individuals to have a marriage annulled.

“This is important to Catholics for whom annulment is permitted by the church, but divorce is not.”

The government’s public consultation on marriage, which is open until 14 June, says: “With the removal of the ban on same-sex couples having a civil marriage, these concepts [of non-consummation and adultery] will apply equally to same-sex and opposite-sex couples and case law may need to develop, over time, a definition as to what constitutes same-sex consummation and same-sex adultery.”

Mr Leigh proposed an amendment to the Civil Partnership Act 2004 which would have allowed cohabiting siblings to have the same legal rights as married couples through the system for gay couples.

Late last year, he argued the new regulations which allow civil partnerships to take place on religious premises for the first time posed a danger to religious bodies which chose not to perform the ceremonies.

He tabled an unsuccessful motion calling for the regulations to be annulled on the basis that they could leave such bodies open to legal challenges if they chose to perform weddings for straight couples but not partnership ceremonies for gay couples, a possibility explicitly addressed in the regulations themselves.

In February last year, he wrote on his website that lifting the ban on marriages for gay couples would “mangle” the institution.

He said: “The British are a tolerant people and it is right that homosexual people should be allowed to get on with their lives.

“But this does not extend to mangling the language of marriage so that, for the sake of the tiny number of gay people who prefer marriage to civil partnership, everyone else in society must have the definition of their own marriage altered forever.

“Once we have departed from the universally understood framework of marriage, there is no logical reason why the new alternative institution should be limited to two people. Why not three? Or thirty-three?”

Just over a fortnight remains for the public to file their response to the government’s consultation on implementing equal civil marriage laws in England and Wales.