The incident involving Katherine Blair is being linked with another attempted robbery nearby half-an-hour earlier. Met Police say: "On both occasions a firearm was seen but not used - no shots were fired. "None of the victims were injured and nothing was stolen during the incidents."
The first incident happened in Boston Place at 8pm involving a female victim and a male suspect.
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Those vying to be named the new Met Police Commissioner will speak to the Home Secretary, London Mayor and Policing Minister.
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The Central and Waterloo and City lines will be affected, with other stations and lines likely to be busier than normal.