They have been slated for causing brain tumours and Alzheimer's, but now it's serious: mobile phones are bad manners.

In some situations, it is generally agreed that starting up a mobile conversation is a dodgy idea (immediately after sex, for instance). But it is now also deemed poor social etiquette to chat on the phone on trains, in bars, or at the theatre.

According to John Morgan, author of the notes in Debrett's New Guide to Etiquette and Modern Manners, diners and train passengers using their mobile "often show an inexcusable lack of consideration towards others around them".

Even worse, individuals who make this phone faux pas risk being taken for "an unsavoury character".

As for receiving a call during a meal or when out for a drink forget it. This sin is almost as serious as leaving a mobile switched on in church.

"I don't see a problem with using a mobile if you are walking along the street," one etiquette expert told The Express. "But if you're in the train there is something about a one-sided conversation which compels you to listen to it and this is robbing you of your mental liberty.

"The most courteous of people do go into the corridor, but I have noticed standards are definitely slipping."

The correct approach when in a restaurant is apparently to abandon mobiles at the reception desk, or with the head waiter - whose job it is to prise the diner from their seat to receive any calls in private.

For those not sure if this strategy would work in McDonalds, never fear - simply switch the mobile off. But do not forget to check messages and to return calls, for that sort of behaviour is a hanging offence in higher circles.

As the guide points out: "Telephone etiquette is rather like tennis: each player has alternate goes at the ball."

But Debrett's also acknowledges that there is no point cursing this scourge on modern manners.

"Those of a more Luddite persuasion should remember that a similar reception greeted the advent of the telephone itself in the 19th century," warned Morgan. "It's arrival was feared by many to signal the end of conversation and of civilised life as it was then lead." ®