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Hundreds more London Underground workers are to go on strike later this week, threatening disruption to Tube services.

The Rail, Maritime and Transport union (RMT) said 500 track patrol staff will walk out for 24 hours from 6.30am on Friday in a dispute over the use of private contractors to “casualise” their jobs.

Around 1,500 maintenance workers are also due to strike at the same time in a separate row over safety.

List of planned Tube strikes 0630 Friday 12th February 2016 and 0629 on Saturday 13th February 2016 0630 and 1829 on Sunday 6th March 2016 0630 Friday 25th March 2016 and 0629 Saturday 26th March 2016 0630 Sunday 27th March 2016 and 0629 Monday 28th March 2016 0630 and 1829 on Sunday 24th April 2016 0630 and 1829 on Sunday 15th May 2016 0630 and 1829 on Sunday 12th June 2016

Track patrol staff walk the tracks looking for faults, often carrying out immediate repairs where and when they can.

The escalation of the strike which originally only included maintenance and renewals engineers dramatically increased the threat of widespread disruption on Friday.

It means if anything goes wrong with the network – a track, signal or power problem – it may not be dealt with until the end of the strike.

The announcement of yet another Tube strike will spark further calls to ban key public transport workers from taking industrial action.

Mick Cash, the RMT union leader, said the track patrol workers’ strike was in protest “over an outrageous attempt to casualise and undermine the jobs of our LU track patrol members through an extension of the use of private contractors. The plan must be halted.

The maintenance and renewals engineers walkout is over “basic safety issues.”

Strikes, consisting of three 24-hour and four 12-hour walkouts, have been spaced out until the middle of June.

Talks at Acas, the conciliation service, continued today to try and avert the maintenance engineers strike after discussions yesterday failed to reach agreement.

A source close to the negotiations said: “The trouble with this dispute is it covers very complex safety related issues – and time is running out.”