The RMT union has announced that Tube drivers on the Piccadilly Line will strike in a row over working conditions.
Tube cleaners and some engineers working for Transport for London will also walk out in separate disputes.
Drivers on Tube lines serving the Hammersmith and City, Metropolitan and Circle Lines voted for industrial action, short of a strike.
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The terror attack carried out by Khalid Masood at Westminster that left four people dead lasted less than two minutes say police.
Two knives were found at the scene in Islington, North London, and the suspects fled on foot before being arrested.
The family of the police officer who was murdered in the Westminster attack have thanked those who were with him 'in his last moments'.