Investigations are continuing into the cause of the Camden crash

Ballot papers have been sent out to 8,000 members of the Rail, Maritime and Transport (RMT) union following derailments near Hammersmith on 17 October and at Camden Town two days later.

Seven people had to be taken to hospital after a carriage hit the tunnel wall at Camden Town and parts of the Northern Line were shut for 11 days.

Crisis talks were held between union representatives and managers last week to try to prevent industrial action.

'Do not feel safe'

Tim O'Toole, managing director of LU, promised to investigate Tube safety, but union leaders said they had "heard nothing" to stop them balloting members.

The RMT opposed the privatisation of Tube maintenance on safety grounds and wants it to be returned to LU.

Bob Crow, RMT general secretary, said: "The people doing the work do not feel that LU is as safe as it should be."

A spokesman for Tube Lines, the private consortium in charge of maintaining the Northern Line, said a report into the Camden crash showed maintenance work met required technical and safety standards.

He said: "The Camden Town area is, unfortunately, a striking example of the immense difficulties we face in renewing and modernising the Tube after decades of under-investment and historically poor decision-making."

The ballot will be over a strike action and a "go-slow" - when drivers could reduce speeds to 15mph - in the run-up to Christmas.