C4M’s high-profile campaign attracted around 668,000 signatures to a petition which said: ‘I support the legal definition of marriage which is the voluntary union for life of one man and one woman to the exclusion of all others. I oppose any attempt to redefine it.’

The law forbids Church of England and Church in Wales churches from conducting same-sex weddings, but other denominations are free to do so if they wish.

The EA’s director of advocacy Dr Dave Landrum said: ‘Even though we are no longer a director of C4M we do remain supportive of the campaign to expose the inconstancy of the legal changes brought by redefining marriage in order to secure protections for those who continue to see marriage as a lifelong union between a man and a woman.’

A spokesman for C4M said: ‘We want to place on record our thanks to our friends at the EA who along with many others helped to establish the largest campaign group in the UK.

‘The EA were not only founding members of C4M, but have been integral to its success.’

He said that C4M would continue to ‘work to protect those individuals, groups and organisations who back traditional marriage and who were denied a voice when these changes were pushed through Parliament.’