Sitel Corp, which runs direct banking and insurance telephone centres, is expecting a boom in business at its sites in London and the Midlands.

BBC industry correspondent Stephen Evans: Concern over 'white collar factories' (51")

The company plans to quadruple the size of its business to £250m ($406m) by 2002 and increase staff from 2,500 to more than 12,000.

Call centres, and their "agents" sitting with headphones in front of computer screens, have become an important part of British service industries.

They already employ 270,000 computer telephonists in the UK - more than coal miners, steel workers and car industry staff combined.

Sitel UK's architect of its expansion plan, Paul Cresswell, said he believed that by 2002 around 2.3% of the country's working population would be employed in call centres.

Wealth to run-down areas

He said: "The average size of call centres will increase as the market further matures and consolidates."

Because the work is all done by telephone, they can be located in cheap areas that are often situated far from the company they are serving.

This gives them the great advantage of bringing wealth and employment to run-down areas in far-flung places.

But a downside is emerging as research is done on this relatively new but important way of working.

'Battery farms'

Academics from the London School of Economics have studied 250 call centres and found levels of monitoring and control on employees that would have been unimaginable to old factory hands.

This has led critics to dub call centres "new sweatshops" and "battery farms".

With constant surveillance from the moment they arrive to the moment they leave the office, most telephonists stay in a job for less than two years, according to the research.