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http://news.bbc.co.uk/1/hi/england/london/4659093.stm

Tube warning after 'bang' heard

Londoners are being urged to avoid the Tube network after reports of an
explosion at Liverpool Street Station.
London Fire Brigade (LFB) said they were called to reports of an
explosion at Liverpool Station in the City of London just after 0900
GMT.

A LFB spokeswoman said four crews were at the scene and more were on
their way to the station which is now closed. Cordons are in place.

A power failure may be behind the problems, Transport for London said.