PinkNews.co.uk has learnt that Labour’s decision to grant its backbenchers a free vote on equal marriage caused a major rift in the shadow cabinet with deputy leader Harriet Harman amongst those calling for the vote to be whipped.

Speaking to PinkNews.co.uk, a Labour source described last week’s meeting of the shadow cabinet as descending into a “massive row” and that two prominent MPs threatened to resign from Labour’s frontbench – if the party went ahead with a whipped vote.

One of the individuals, a Labour parliamentary veteran, held top cabinet positions in both Gordon Brown and Tony Blair’s governments, and both MPs are of deep religious faith.

However, it’s understood Labour’s deputy leader Harriet Harman, Angela Eagle, shadow leader of the House of Commons, and shadow education secretary Stephen Twigg disagreed with allowing equal marriage to become a conscience matter.

Meanwhile, backbench Labour MPs Mary Glindon, Jim Dobbin and Joe Benton have today appeared in a list of parliamentarians who have signed an open letter against the government’s marriage reforms.

Last Tuesday, leader Ed Miliband confirmed Labour backbench MPs would be given a free vote over the government’s marriage proposals.

Mr Miliband claimed: “The whole shadow cabinet is united in supporting same-sex marriage together with the vast majority of all Labour MPs.”

Labour had previously indicated it would impose a three-line whip and the party’s affiliated LGBT campaign group criticised the U-turn, saying:

“LGBT Labour firmly believes issues of equality are not matters of conscience but fundamental rights. We believe this position has the strong support of Labour members and activists who also share our disappointment.”

Labour MPs Angela Eagle and Stephen Twig are both patrons of LGBT Labour.

Today’s revelations indicate that achieving 100% unanimous support for equal marriage in Mr Miliband’s front team could be a tall order.

In public the change in position by Labour has been attributed to the government’s decision to incorporate religious ceremonies in its equal marriage plans.

However, a Labour source told PinkNews.co.uk “they are going to use the new religious freedom angle as an excuse for not whipping.”

The Scottish Labour Party is understood to have been debating whether or not to whip the vote on equal marriage at Holyrood as well.

Members are now concerned that opponents of equal marriage might use Labour’s decision at Westminster as an excuse not to whip in Scotland either.