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.

The RMT has called a series of seven strikes in the coming months over the maintenance dispute, although talks are continuing at the conciliation service Acas.

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The union accused London Underground (LU) of trying to "bulldoze through" the training of contract staff to undertake track patrolling duties in a "direct attack" on the job security of RMT's track patrol members.

General secretary Mick Cash said: "This dispute is over an outrageous attempt to casualise and undermine the jobs of our London Underground track patrol members through an extension of the use of private contractors. The plan must be halted.

"RMT will continue to fight all attempts to undermine and casualise jobs across London Underground as the company looks to cut corners to meet Government austerity targets.

"This union will not accept the undermining of jobs, conditions and safety or the creeping privatisation of functions."

Steve Griffiths, chief operating officer at London Underground, said: "When we introduce the night Tube we will need some track patrol staff working on Sunday nights rather than Fridays.

"We've given our existing staff the choice whether or not to cover these shifts, which protects their work/life balance.

"No jobs are at risk and contract staff will only be used temporarily, to cover shifts that our employees choose not to work.

"This freedom to choose for our staff is part of our pay and night Tube offer, which the RMT has recommended its members accept. We'll be doing everything we can to ensure this doesn't impact services for customers."