Bishops of the Church of England are being urged by their flock to turn out en masse on Wednesday for the Lords debate on equal rights for gay couples wishing to adopt.

In an open letter sent to all the diocesan bishops of the Church, more than one fifth of the lay members of the General Synod urge the 26 bishops in the Lords to help to overturn the Sexual Orientation Regulations at its final vote.

Many peers and MPs from across all parties are unhappy with the way the changes to adoption law have been processed through Parliament. Hundreds of Christians are expected to turn up for a peaceful protest vigil outside Parliament on Wednesday during the debate.

The Roman Catholic Church has led the campaign against the regulations, which could put its adoption agencies out of business because it would flout Catholic teaching for them to accept public funding to facilitate gay adoption.

Protesters also fear the regulations will compromise the teaching programmes in faith schools, which will not be exempt. They are warning of “substantial danger” that it will be illegal under the regulations for faith schools to continue to teach that sex outside marriage is wrong.

If the bishops are successful in persuading the Lords to defeat the regulations, it is certain to fuel the Government’s determination to press for a 100 per cent elected chamber.

If bishops were included, it would be with a nominal role only, without voting rights.

The former Archbishop of Canterbury, Lord Carey, told The Times that if the Lords reforms went ahead on this basis, the Church of England should press for disestablishment.

In their letter, more than 40 members of the General Synod quote the present Archbishop, Dr Rowan Williams, as saying: “We in the UK do not have anything like this history of top-down rule by regulation.

“We have in practice taken for granted that the State is not the source of morality and legitimacy but a system that brokers, mediates and attempts to coordinate the moral resources of those specific communities, the merely local and the credal or issue-focused, which actually make up the national unit.

“This is a ‘secular’ system in the sense that it does not impose legal and civil disabilities on any one religious body; but it is not secular in the sense of giving some kind of privilege to a nonreligious or antireligious set of commitments or policies. Moving towards the latter would change our political culture more radically than we imagine.”

The lay members continue: “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.”The regulations were dealt with last week by a House of Commons committee of 16 MPs, which met for 90 minutes. Christian protesters are complaining that even the MPs on the committee itself had been appointed just 15 hours before it met and the room arranged for the debate was so small that there was not enough room for all the MPs.

Observers present reported that the meeting started in confusion and that only four MPs were allowed to speak on the regulations before the vote was passed in favour. Eleanor Laing, the Tory MP, supported the Regulations and said that “her brand of Christianity” preached “live and let live”.

On Wednesday Baroness Andrews will move that the draft regulations be approved.

The aim of the regulations is to outlaw discrimination on the grounds of sexual orientation in relation to providing goods, services, facilities, premises, education and public functions.