PlayThe Government will review ministerial security in the wake of a protester's attack on Business Secretary Lord Mandelson.
A cup of green custard was thrown over the minister by an anti-Heathrow protester as he arrived at a conference on green energy.
Leila Deen, 29, of the campaign group Plane Stupid, launched her attack outside the Royal Society before walking away unchallenged.
She told reporters afterwards that she was protesting over a reported meeting between Lord Mandelson and lobbyists for Heathrow owner BAA.
A Metropolitan Police spokesman said: "Police are investigating circumstances surrounding an incident in Carlton House Terrace, SW1, at about 8am today.
"Officers from Westminster CID investigate. There have been no arrests. No complaint has been received."
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