The Bishop of Grantham has become the first Anglican bishop to declare publicly he is in a gay relationship.

The Right Rev Nicholas Chamberlain said in an interview with the Guardian on Friday that his sexuality was no secret and was known by both his senior bishop and the Archbishop of Canterbury Justin Welby when he was appointed last November. He told the paper he had not intended to speak out but was forced to when a Sunday newspaper threatened to expose him.

"People know I'm gay, but it's not the first thing I'd say to anyone," he told the Guardian. Sexuality is part of who I am, but it's my ministry that I want to focus on."

Welby said he was "fully aware of Bishop Nick's long-term, committed relationship". In a statement he said the appointment to the episcopate 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."

Chamberlain insisted his relationship was within church guidelines that stipulate gay clergy cannot marry and must remain celibate. He said that during the appointment process, "we explored what it would mean for me as a bishop to be living within those guidelines."

The Bishop of Lincoln, the Right Rev Christopher Lowson, made the appointment and was backed by Welby, according to the Guardian interview. Lowson said in a letter to his parishes that he was satisfied Chamberlain "fully understands and lives by" the Church's rules for gay clergy. "The fact that Bishop Nicholas is gay is not, and has never been, a determining factor."

A spokesman for the Church of England said: "The church has said for some time that it would be unjust to exclude from consideration for the episcopate anyone seeking to live fully in conformity with the church's teaching on sexual ethics or other areas of personal life and discipline."

The revelation is a sign of the shift within the Church of England. In 2003 the failure to appoint Jeffrey John, who was openly in a same-sex relationship, as Bishop of Reading became one of the hallmarks of Rowan Williams' tenure as Archbishop of Canterbury. Despite John's insistence the relationship was celibate his nomination was withdrawn after being initially backed by Williams.

Chamberlain said he did not want to become "the gay bishop". But he acknowledged he would 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 added: "It's not to say this isn't an important matter – I'm not brushing it aside." He went on to say the church needed to concentrate on other issues such as poverty and refugees.

The Church's House of Bishops - a senior decision making body - will meet later this month with Chamberlain in attendence. One of the primary issues will be whether to suggest some form of acceptance like a blessing or prayer for gay couples within the Church. Such a move would cause an outcry among conservative but many on the liberal wing of the C of E see it as a long overdue.