John Doyle, Jubilee line general manager for London Underground: said:
“The unions have informed us that they are in dispute with us over what they perceive as a breakdown in industrial relations. We are keen to understand their concerns and so will be meeting with the unions later this week to discuss these issues in greater detail.”
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Paramedics were on standby outside the police cordon as armed officers stood by the property.
A team of researchers has taken samples from the ten busiest London Underground lines to work out which are the dirtiest.
The action comes on top of an emergency timetable which has seen hundreds of trains a day being axed.