The Waterloo & City Line will close next April

Managing director Tim O'Toole said the work, to include replacing the track, will in the long-term mean reduced journey times and refurbished trains.

Maintenance firms Metronet and Tube Lines must replace a third of all tracks on the network by 2010 under the Public Private Partnership.

The upgrading work will begin on 1 April 2006.

Mr O'Toole said: "The Waterloo & City line suffers from constant failures and urgently needs upgrading."

He said they had agreed to the request from Metronet so they could deliver "improved reliability, increased passenger capacity, quicker journey times and a smoother ride".

After consultation, LU said it had decided the line should close for five months so the improvements could be made all at once rather than over a succession of weekend closures.

London Transport Users' Committee chairman Brian Cooke said: "If the Underground is to improve, then on occasions there will have to be short-term pain to deliver long-term gain."

Detailed travel advice will be issued for passengers before the work starts.