Labour tensions over gay marriage have surfaced as MPs attacked the Government’s plans for same-sex weddings and shadow ministers clashed over their tactics on the issue.

Ed Miliband has decided to allow his MPs a free vote on the plans when they are introduced in the Commons next month, but all members of the shadow Cabinet are to support the reform.

Although attention has focused on Conservative divisions over marriage equality, the subject is also causing anguish within sections of the Parliamentary Labour Party.

Three MPs – Joe Benton, Jim Dobbin and Mary Glindon – and the Labour peer Lord Anderson of Swansea signed an open letter protesting that the Government had no mandate to “redefine marriage”.

Other senior Labour MPs, including frontbench spokesmen, are expected to vote against the proposals out of religious conviction.

Differences of opinion over tactics also emerged at the last shadow Cabinet meeting, with prominent figures, including the deputy leader Harriet Harman, opposing a free vote and arguing for it to be whipped.

She was supported by Stephen Twigg and Angela Eagle, who are both patrons of the Labour Campaign for Lesbian, Gay, Bisexual and Transgender Rights.

However, Mr Miliband argued that the issue was a matter of conscience and “religious principle” and should be subject to a free vote for MPs other than shadow Cabinet members.

The PinkNews.co.uk website reported yesterday that two unnamed shadow ministers threatened to resign if the party went ahead with a whipped vote.

Labour sources denied the report and insisted there had been no “row” over the subject, but confirmed there were “differences of opinion” and “strong feelings” over the case for same-sex marriage.

Thirty-five Tory MPs were among the signatories to yesterday’s letter, illustrating the depth and strength of hostility to David Cameron’s plans within his own ranks. They included the former ministers Tim Loughton, Sir Gerald Howarth, Tim Loughton and David Davis.