The Mayor's Senior Advisor for Volunteering, Charities and Sponsorship, Veronica Wadley, said:
"2012 has been an extraordinary year for London and the achievement of hosting the Games has shown what a terrific welcome we can give visitors. With the help of the Team London Ambassadors and other volunteering programmes, we want to build on the city's reputation for being able to deliver the most fantastic place in the world to visit."
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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.