Gay Tory MPs who hide their sexuality will risk Conservative party unity by voting against the prime minister's same-sex marriage plan, it has been claimed.

David Cameron is expecting around 180 Conservative MPs to refuse to support him when MPs get their first vote on gay marriage.

It is predicted that 120 will vote 'no', with a further 60 abstaining altogether. Given this opposition, the support of Labour and Liberal Democrats will be necessary for Cameron to win the vote.

But some rebel Conservatives have been warned they face accusations of hypocrisy by opposing the bill, because they are secretly involved in gay relationships. Their stance may even result in them being outed against their will, according to prominent Conservative pundit Ian Dale.

Dale, who is in a civil partnership with a man, viewed a list of Conservative opponents to gay marriage.

He said: "I note with interest the names of several MPs who most people in the Westminster village known to be close-case gays. And I also note the names of two supposedly straight MPs who I know to be conducting gay affairs.

"I don't believe in outing anyone. Because of the rank hypocrisy there will be others who will take a different view . How is it possible to be married and yet at the same time vote to deny that privilege to someone whose pants you have just pulled down?"

Senior Tory cabinet members have urged the party in parliament to united behind the Marriage (Same Sex Couples) Bill. Chancellor George Osborne, Foreign Secretary William Hague and Home Secretary Theresa May claimed gay marriage was supported by "a substantial majority" of British people.

Almost all Liberal Democrats are expected to vote in favour of same-sex marriage. On the Opposition benches, it is predicted that about 20 Labour MPs could vote against the bill or abstain. Three shadow ministers are expect not to vote in favour. They are Rob Flello, Gavin Shuker and Stephen Timms.

Cameron faces grassroots unrest on the issue. Opponents predict the Conservatives could lose votes at the next election over same-sex relationships - which has been compared to the party's tortured relationship with Europe.

More than 20 current or former Conservative constituency chairmen wrote to Downing Street,claiming that "significant damage" would be done to the party by forcing through gay marriage. One chairman called the move "reckless."