It will be transferred to the private sector and reclassified as an overground line once an extensive upgrade is completed in 2010.

The Rail Maritime and Transport union claims this will compromise safety.

But Transport for London (TfL) said it will improve frequency, comfort, safety and security for passengers.

'Wrong judgement'

The RMT said public private partnership on the London Underground has already created a fragmented network with frequent service disruptions.

A statement said: "Privatisation will exacerbate that fragmentation and could be the thin end of the wedge with other lines and services being threatened with sell-off in future."

TfL maintained that "unlike other railways, the new system will effectively be under public control due to tight restrictions imposed on the contractor".

A total of �1.4bn is to be invested to upgrade trains and stations, improving staffing levels, increasing the frequency of services, installing CCTV and emergency help points.