No further talks between leaders of the RailMaritime and Transport (RMT) union and London Underground leaders have beenarranged, raising the threat of disruption to Tube services next week.
Earlier this week RMT officials said its members will walk out at 4pm on Tuesday until the same time on Friday.
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Boris Johnson and Amber Rudd have both criticised social media giants for failing to detect and remove extremist material online.