Coalition for Marriage (C4M), the UK lobby group that opposes marriage rights for gay couples, has delivered 46,000 leaflets in the constituency of Corby, which is due to hold a by-election on Thursday.

The by-election was triggered by the resignation of Conservative MP Louise Mensch in August.

According to the Scotsman newspaper, C4M have dropped 46,000 leaflets through voters’ doors arguing against the coalition government’s plans to legalise civil marriage equality before the 2015 general election.

It is claimed by Tories insiders that UKIP, who are opposed to marriage equality, may succeed in taking a vital share of the Conservative’s core vote.

However, Corby is seen as a marginal and political commentators say it should be an easy win for Labour.

On Tuesday, Chancellor George Osborne urged the Conservative Party to embrace the reform, citing last week’s equal marriage victories in the US.

Mr Osborne said the Republicans had lost “swathes of voters” because of their opposition to equal marriage and abortion rights for women.

Colin Hart, campaign director of C4M, responded by saying: “Yet again the government’s spin doctors are trying to claim that redefining marriage is a vote winner. Quite the opposite is true. In the US, voters in 31 states have backed traditional marriage at the ballot box, often by large margins and as recently as May 2012 in North Carolina”.

He added: “The four results by the narrowest of margins in liberal states last week are not representative of US public opinion”.