Vicars rarely grumble about people joining their congregations, but a number might be dreading a visit next month by a "mystery worshipper" - the Church equivalent of the restaurant critic.
In an unprecedented initiative, as many as 100 specially recruited "researchers" will turn up incognito in pews across London on April 24 in order to judge the quality of the Sunday service on offer.
The volunteers, using pen names such as "Church Mouse" and "Dunelm" to protect their anonymity, will then post their verdicts on the Christian website at www.Ship-of-Fools.com.
The exercise has been inspired by the market research techniques of supermarket chains such as Asda and Tesco, which send "mystery shoppers" covertly into their own stores to gather consumer feedback.
The reports, which could make uncomfortable reading for some, will be published simultaneously on the opening day of the National Christian Resources Exhibition in Surrey in May.
Churches will be judged on the warmth of the welcome, the length and content of the sermon, the music and even the after-service coffee and the softness of the pews.
Among other things, mystery worshippers will be asked to describe the moment which brought them closest to heaven and the moment closest to "the other place".
Ecclesiastical "Oscars" will be awarded in categories such as best sermon, best use of music and best overall church.
Simon Jenkins, the editor of the website, said: "Our volunteers go to a church they have never been to before, and experience the service as an outsider.
"For the church being visited, the only clue is the calling card dropped discreetly into the collection plate, bearing the picture of a masked man in Lone Ranger pose."
The project began seven years ago, and the 1,000th report will be published next week on Palm Sunday. Reviews have come in from as far afield as Bethlehem and Bangkok, but never before has a city-wide inspection been attempted.
"Many churches have taken criticism well," said Mr Jenkins. "One minister posted the review on a notice board with a 'we must improve' note.
"Another read the entire report from his pulpit. I also know of a church which took the review on a weekend retreat and the congregation mulled over how it could best answer the criticisms levelled."
Stephen Goddard, the co-editor of the website, said: "Whether they are happy-clappy, bells and smells or rock the flock, we'll have a better picture of what neighbouring churches were like in one 24-hour period."
One mystery worshipper who will be taking part in next month's survey, using the pseudonym "Holy Stone", said: "It won't have quite the shock value of the normal reviews because the clergy know we are coming. But to cover a whole area like this will be very revealing."
Among examples of previous reviews is one about St Bartholemew's Church in Brighton by a mystery worshipper calling themself "Corpus Cani".
When asked whether the service made him or her feel glad to be a Christian, the reviewer said: " It was rather like a one-night stand: Great at the time, but you don't want to hang around after the cigarette.
"Not recommended as a first experience of Christianity, but for the old hand it is a reminder of the glory of the Resurrection."
In another covert inspection report, on Ginghamsburg church in Tipp City, Ohio, USA, "The Preacher Man" gave his views on the pre-service atmosphere: "I arrived a few minutes late. I found the worship band in full swing on an upbeat, loud, almost raucous, Christianised version of the Ricky Martin pop song Livin' La Vida Loca.
"I was distracted by the music (the Ricky Martin song baptised with Christian lyrics about the Resurrection).
"But as I listened, I couldn't get Ricky's scantily-clad dancers out of my mind. I half expected one to come twirling across the stage."