Police are investigating a suspect car parked outside Territorial Army barracks on Stonecot Hill near Sutton. The red Volvo has been parked outside for at least 24 hours. Specialist officers have been called to the scene.
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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.