The Law Society is facing legal action from a religious lobbying group over its decision to call off a conference that was set to debate gay marriage at the society's headquarters.

Christian Concern – which has instructed Shropshire-based AndrewsLaw Solicitors and human rights barrister Paul Diamond – has sent a letter before action to the Law Society alleging breach of contract.

The event, which was set up by US organisation World Congress of Families in partnership with the Christian group, was set to be held at the Law Society's London base on 23 May. However, the society cancelled the event due to a conflict with its "role in promoting diversity."

The event, titled "One man. One woman. Making the case for marriage, for the good of society", was set to debate "the issues surrounding the Government's proposed redefinition of marriage", including its plans to allow same-sex marriages.

A subsequent booking at the Queen Elizabeth II Conference Centre in Westminster was also later cancelled.

Christian Concern's letter before action states: "Our client wishes to make a claim against the Secretary of State for Communities and Local Government, acting through the Queen Elizabeth II Conference Centre, in view of your breach of contract on the booking of the above mentioned event.

"We require a response within seven days of today's date, and in the absence of any satisfactory conclusion being reached in connection with our client's concerns, court action will be commenced."

The Law Society is taking advice from human rights specialist law firm Bindmans.

In a statement, Christian Concern CEO Andrea Minichiello Williams said: "The implication of the decision to cancel this booking is that belief in marriage between a man and a woman is offensive and that those who hold such an opinion have to be treated less favourably as a result.

"There is no justification in contract, or in law generally, for the Law Society or the QEII Centre to hold this position, and their termination of their contracts with Christian Concern is a breach of the terms of the contract."

"Gay rights' group Stonewall has recently held its conferences in the Law Society and the QEII Centre attended by government ministers. These conferences talk about promoting 'equality and diversity'. Yet it would seem that neither the Law Society or the QEII Centre extends the same hospitality to Christian groups."