Mr Crow is awaiting a letter detailing plans to improve safety

Rail, Maritime and Transport (RMT) union members plan to drive at no more than 25mph on some sections of track.

But RMT general secretary Bob Crow is "hopeful" the action will be called off after meeting London Underground managing director Tim O'Toole.

He is now awaiting a letter promising more rigorous safety inspections.

The dispute follows two derailments in October at Hammersmith and Camden Town.

Mr Crow said: "It is encouraging that the employers appear to have conceded the case for increasing the level of track inspections and other standards.

Progress made

"On the basis of what we heard this afternoon we are hopeful that progress can be made, but until the RMT executive has a chance to see the proposals in writing, next week's safety go-slow remains on."

Mr O'Toole applauded the "constructive" approach taken by the unions and the private firms that maintain the network after Thursday's meeting.

Thursday's talks between Mr Crow and LU managing director Tim O'Toole follow an informal meeting between the two in the early hours of Wednesday.

They talked for two hours on the track near Hammersmith, while maintenance inspections were going on.

Defective track

Mr Crow had said after the trackside meeting with Mr O'Toole that the frequency of maintenance inspections, lighting and other requirements fell "well short" of those needed to run a safe system.

The union is campaigning for maintenance work to be taken away from private firms and given back to LU.

He said it was up to LU and the private firms maintaining the system to show they were serious about improving safety.

Meanwhile, a track defect caused delays on three lines on Thursday as trains had to be driven at 5mph over a section near Euston Square.

LU said the problem was discovered during a routine line inspection.

The defect led to delays on the Metropolitan, Circle and Hammersmith and City lines - used by 600,000 travellers a day.