Actors' voices are their most precious tools and many train for years honing their mellifluous tones so they can project to the furthest corners of a theatre.

But at the National Theatre actors are getting a little help - in the form of microphones - to boost their performance.

The news has outraged actors and voice teachers who believe that it compromises an unwritten artistic convention and crosses the boundary between theatre and television.

Although microphones are often used in musicals they are less accepted in straight theatre.

'Lazy actors'

The first time the system was used at the National was two years ago - only it seems that no-one found out until now.

Graham Sheffield, the artistic director of the Barbican said to the Guardian newspaper, the microphones could "become a general support mechanism for lazy actors".

Trevor Nunn, the artistic director of the National Theatre and the man who authorised the 'miking up', has fiercely defended his decision to take advantage of technology.

Writing in the Daily Telegraph - the paper that first broke the story in horror - Mr Nunn reveals what he calls "the best-kept secret" at the National, that is the appalling acoustics of the Olivier Theatre, its largest space.

"Each generation of actors and directors working at the Olivier has had to confront these problems," he said.

'Acoustic enhancement system'

First Nunn decided to install a special "acoustic enhancement system".

"Even after this innovation complaints concerning audibility were received on a regular basis, despite the expert vocal training constantly undertaken in the theatre," he said.

Finally the actors got their own tiny radio microphones. Nunn said his changes "have almost nothing to do with amplification" and the maximum amount any actor is boosted is by 10 per cent.

'Complaints almost stopped'

While those in the theatre industry may have reservations it seems that theatre audiences are much happier. According to Nunn complaints about audibility in the theatre have almost completely stopped.

Despite the theatrical outrage Nunn is planning to keep the microphones.

Fiona Walsh of the National said: "In no way is this going to replace the importance of voice work.

"It's an experiment really and Trevor is planning to use the microphones in the next two productions in the Olivier."