Tube drivers have announced they are going on strike on the first working day after the Easter weekend.
The strike on the Jubilee Line is the first of two over working conditions and claims of bullying.
Paul Brand has the latest from Transport for London's headquarters.
More top news
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.