Peter Tatchell, the homosexual rights campaigner accused of "indecent behaviour in a church", is charged under section 2 of the Ecclesiastical Courts Jurisdiction Act 1860.

That law itself is derived from a much older one, the Brawling Act of 1551.

Canterbury magistrates' court, where Mr Tatchell's case is being heard, was offered a definition of "indecent behaviour" by the prosecuting lawyer, Robert Montague.

"The term bears no sexual connotation", Mr Montague told the court. "Section 2 of the act does forbid what it calls riotous, violent or indecent behaviour in a church.

Peter Tatchell: One of the most prominent homosexual rights campaigners

"But the way I would describe Peter Tatchell's conduct is that it was inappropriate, unseemly or indecorous."

The case against Mr Tatchell follows an incident last Easter Day, 12 April, when he and several colleagues from the campaign group OutRage! climbed up into the pulpit of Canterbury Cathedral during a service.

They interrupted the Archbishop, Dr George Carey, who was preaching his Easter sermon, in order to protest at what they called his support for discrimination against lesbians and gay men.

In 1860 - and even more in 1551 - the law was used to prevent any disturbance to a church service.

It was used much more recently than that to punish people who used a church to voice their criticisms.

In 1966 the Labour party was holding its annual conference in Blackpool. As usual, a church service was held early in the proceedings.

Several protesters, angry at the Labour government's stand on Vietnam, interrupted the service by calling out: "Oh you hypocrites ! How can you use the word of God to justify your policies ?"

One of the group was sentenced to two months in prison, the maximum sentence (with a fine of £200) possible under the act.

But some prominent secular critics, and a few Christians, say the act should be scrapped.

They have signed an appeal organised by the National Secular Society, a humanist group, which calls for the act to be repealed.

Signatories include the journalist Polly Toynbee, the writer Alan Bennett, and the former Labour leader Michael Foot.

They are concerned that the act gives the church a unique degree of protection, and an immunity to criticism which no other institution enjoys.

They say that if Peter Tatchell and his group had protested anywhere except in a church, then he could have been charged only with a public order offence, with probably a lesser penalty.

If the act is not repealed, its critics say, then it should be extended to include secular ceremonies as well, and the maximum sentence should be a fine, not imprisonment.

In 1860 the church was an important part of the British establishment, a focus of authority whose priests certainly did not expect to be interrupted or criticised in their own places of worship.

In 1998 Britain is a different society, with a different church - one that is less sure of its place in the national scheme of things, and more used to hearing critical voices raised against it.

Sending people to prison for interrupting archbishops - even if the interruption came as a rude shock to many people - no longer seems quite as natural a punishment as it used to.

And it could be a way of making martyrs - not something today's church does very often.