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Hardline Catholics are trying to kick MPs and peers out of their church if they vote in favour of gay marriage.

Campaigners at Catholic Truth have called for Catholic politicians at Westminster to be banned from receiving Holy Communion and “excommunicated” if they back same-sex weddings.

They have sent a series of emails to Catholic MPs and peers just days before a parliamentary vote on the Government’s Bill to legalise gay marriage.

But MPs have branded their actions “aggressive” and “offensive” and is urging them to stop their campaign.

Labour MP Rob Flello, who is a Catholic, said: “I was horrified that members of our church should be openly vilified for taking a view which may be different to my own but is one that was based on their own conscience,”

“It doesn’t help people on my side of the argument that this organisation is taking such a hostile, aggressive position.

"It does not exactly seem in keeping with my view of what the Christian faith is all about.”

Catholic Truth is behind an online petition which describes politicians who support same-sex marriage as “manifest public sinners”.

The document has been signed by more than 400 people and urges the Vatican to ensure that no Catholic church will serve Communion to politicians who have backed “abortion, euthanasia or same-sex marriage”.

The Catholic Legislators’ Network, which represents Catholics in the Houses of Parliament, said Catholic Truth’s correspondence had been received by several politicians.

The Network’s secretary Chris Whitehouse said: “The communications from this organisation have been aggressive, confrontational and condemnatory. They have caused deep and profound offence.”

Patricia McKeever, editor of Catholic Truth’s newsletter, said the group was just urging the Vatican to support Catholic Church laws.

She said: “This is not a question of belief or opinion – it is simply a matter of Canon Law,” she said.

“By supporting this policy [gay marriage] these politicians have effectively excommunicated themselves.”