The relevance of Church of England Bishops as the "custodians of moral and ethical values" in debates in the House of Lords is to be put to the test this Wednesday.

Forty-two members of the General Synod of the Church of England have issued the challenge to their national leaders as the Government considers a fully elected second chamber, and whether the 26 bishops of the state church should keep their exclusive places on the coveted red benches.

In a letter signed by lay members of the Church's ruling body, the bishops were told that the arguments for retaining the unique privileges enjoyed by the Church of England in the upper house would be severely negated unless the bishops, enblock, turned up to vote against the introduction of the Equality Act (Sexual Orientation Regulations) 2007 when debated by the Lords on Wednesday.

The Government claims the regulations will simply prevent discrimination in the provision of services against people on the grounds of their sexual orientation. However, opponents widely believe the regulations will lead to the promotion of homosexuality in primary schools and the 'silencing' of Christians who believe in the Bible's teaching on relationships.

Bishops were told in the letter over the weekend: "Given the great significance of this vote, many people would understand that the responsibility that bishops undertake as members of the House of Lords requires them on such occasions to vary their crowded timetable in order to attend the debate. Many Christians will be praying outside Parliament at the same time, giving up other activities that could rightly claim their attention.

"We also note the spirited defence made last week of the role of the bishops in the House of Lords by the Archbishop of York and the Bishop of Chelmsford. Important substance would be given to their words if all the bishops in the Lords were to attend to vote."

Anthony Archer, author of the letter and a member of the Crown Nominations Committee said: "This is a rare opportunity for all bishops to unite around this subject. It is also an outstanding moment for them to make a clear statement about their role in the House of Lords in the context of the proposals for reform, which looks increasingly likely to lead to a diminishing of their ability to be the conscience of the nation in our democratic process."