A conservative Anglican group has called the appointment of a man as a bishop who later came out as gay a “major error”.

The Bishop of Grantham yesterday became the first Church of England bishop to come out as being in a same-sex relationship.

Nicholas Chamberlain, who is the bishop for Margret Thatcher’s childhood hometown, has come out after he said that a Sunday newspaper was planning to out him. Mr Chamberlain said that it was no secret that he has been in a long-term, albeit celibate relationship for many years.

In an interview with The Guardian, Mr Chamberlain said: “It was not my decision to make a big thing about coming out.” He added: “People know I’m gay, but it’s not the first thing I’d say to anyone. Sexuality is part of who I am, but it’s my ministry that I want to focus on.” He also said that those appointing him a bishop just six months ago knew his sexuality. “I was myself,” he said. “Those making the appointment knew about my sexual identity.”

In a statement, the Archbishop of Canterbury Justin Welby said he was already aware that the Bishop was in a same-sex relationship.

He: “I am and have been fully aware of Bishop Nick’s long-term, committed relationship. His appointment as bishop of Grantham was made on the basis of his skills and calling to serve the church in the diocese of Lincoln. He lives within the bishops’ guidelines and his sexuality is completely irrelevant to his office.”

But now conservative Anglican group Gafcon has released a statement referring to the appointment of Bishop Chamberlain as a “major error”.

“There are aspects of this appointment which are a serious cause for concern for biblically orthodox Anglicans around the world, and therefore we believe that this appointment is a major error,” the Gafcon statement said.

The letter was signed by the orgnisation’s general secretary, the Most Reverend Peter Jensen, and Canon Andy Lines, the chairman of the group’s UK task force.

It goes on to say the “element of secrecy” in the new role implied “that it has been arranged with the aim of presenting the Church with a ‘fait accompli’, rather than engaging with possible opposition”.

The bishop said that his relationship is “faithful, loving, we are like-minded, we enjoy each other’s company and we share each other’s life.”

While he says that he doesn’t want to just be known as the “gay bishop”, he will stand up for LGBT rights at meetings of the Council of Bishops, which will meet shortly to discuss the issue of homosexuality. “I will speak [at the meeting], and this part of me will be known. I hope I’ll be able to be a standard-bearer for all people as a gay man. And I really hope that I’ll be able to help us move on beyond matters of sexuality,” he said. “It’s not to say this isn’t an important matter – I’m not brushing it aside.”

The Church of England has faced increased tensions over LGBT rights in the past few years following to the introduction of same-sex marriage in England.

Under hastily-implemented rules, the Church of England remains opposed to same-sex weddings – meaning that clergy cannot carry out services or ‘blessings’ for same-sex couples.

The rules also ensure that gay members of the clergy are banned from getting married themselves.

The policy has seen a number of gay vicars punished simply for getting married – with some hiding in fear while others were dismissed from their jobs.

A hospital chaplain who brought an employment tribunal case against the Church on the issue recently lost out – after the Church successfully argued it was exempt from the parts of the 2010 Equality Act that ban discrimination on the grounds of sexual orientation.

However, current policy is seen as largely unsustainable, with members of the clergy continuing to flout the rules by performing blessings or entering same-sex marriages themselves.

Evangelicals within the Church have threatened a split if the policy is relaxed, while pro-LGBT clergy have threatened action if it is not relaxed.

The Archbishop of Canterbury has largely attempted to ensure the Church pleases both groups, insisting the Church must always reach out to people who believe homosexual relationships are “deeply wrong”