A community campaign group says work to bring Tube trains south of the Thames must be a priority.

The Sydenham Society, which has been campaigning for better public services for 30 years, says it is time to stop talking and start building.

Work on the East London line extension was scheduled to start early this year after the Government gave the plans the green light in October.

But a delay north of the river could put the 2006 completion date back another 18 months.

Sydenham Society chairman Pat Trembath said: "We need this line to improve communication and regeneration. It is such a positive step and would be a major boost for the area."

Plans suffered a setback recently when Braithwaite viaduct, in Bishopsgate, near Liverpool Street station, was awarded Grade II-listed status, halting construction in the area.

Mrs Trembath says this should not affect work south of the river, adding: "The voice of south London says, Get on with it'."

The 1,000 member-strong society has contacted community groups across south London and has called on local councillors and MPs to press for action.

Councillor Gavin Moore said: "I very much welcome the society's support for this vital transport link."

He added: "Everyone would like to see a budget agreed and a clear decision made."

London Underground confirmed its commitment to the project, which it is hoped will attract £600m of private money.

But a spokesman for Greenwich and Lewisham GLA member Len Duvall says work is not far enough advanced for the southern extension to continue while permission is sought to build near the listed viaduct. The East London line will open up the Underground map through Brockley and Sydenham as far as Croydon.

It will link up with the existing East London line to Hackney through the City.