The Rt Rev Richard Holloway, Bishop of Edinburgh and Primus of the Episcopal Church, has submitted a statement supporting Mr Tatchell, who is charged with "indecent behaviour in a church". His evidence will put him in direct conflict with his opposite number in England, the Archbishop of Canterbury, Dr George Carey.
Mr Tatchell interrupted Dr Carey's sermon,with six other protesters from OutRage, but he was the only one charged. He walked into the pulpit and addressed the congregation, criticising Dr Carey's support for discrimination against lesbian and gay people.
The prosecution claims that any form of protest in a church is "indecent" under the Ecclesiastical Courts Jurisdiction Act 1860. Mr Tatchell intends to challenge the way the Act gives the church "privileged protection against protest".
More than 700 lawyers, professors, clergy and human rights activists have signed a petition by the National Secular Society deploring the prosecution of Mr Tatchell and calling for the Act to be repealed on the basis that "it gives the church unique and sweeping powers to suppress dissent".
Among the signatories are Sir Ludovic Kennedy, Bishop Derek Rawcliffe, Alan Bennett, Vanessa Redgrave and Paul Foot. The National Secular Society wrote to Dr Carey in October, urging him to call for the charge to be dropped or at least ask the court not to jail Mr Tatchell. However, Dr Carey refused. The court turned down Mr Tatchell's request for permission to summon Dr Carey as a witness, but declined to give a reason.
Mr Tatchell, who has been involved in more than 1,000protests during 30 years campaigning for lesbian and gay human rights, said last night: "Dr Carey calls for compassion to be shown to a human rights abuser, General Pinochet, but not for a human rights defender like Peter Tatchell."
The veteran Labour left-winger, Tony Benn, will tomorrow give evidence at Mr Tatchell's trial, at Canterbury Magistrates' Court. He branded as "obsolete" the Ecclesiastical Courts Jurisdiction Act 1860 - which was formerly part of the Brawling Act of 1551.
"People don't realise how very, very deep feudal control is in our society," said the MP for Chesterfield. "When you think that Major appoints an archbishop who is then protected by a brawling act you realise we haven't escaped much from the medieval repression of activities and ideas."
Bishop Holloway said in his statement: "Peter Tatchell's tactics do not attract much sympathy from the general public and I do not always approve of them myself."
But, he added, they have "a certain historic precedent.
"History shows us that reform movements always gain their energy from the extreme actions of a few individuals who feel they have been abused by the system against which they are protesting."