A terror suspect who escaped surveillance by changing into a burka on a visit to a mosque was on the run on Sunday night.
Mohammed Ahmed Mohamed was last seen at a west London mosque on Friday afternoon.
The 27-year-old is "not considered at this time to represent a direct threat to the public", Scotland Yard said, but urged anyone who sees him to call 999 immediately.
Mr Mohamed entered the building wearing Western-style clothes, but CCTV images issued by Scotland Yard later show him leaving with his face and body fully covered by the traditional Islamic garment.
He is subject to a Terrorism Prevention and Investigation Measures notice (Tpim).
A court-imposed anonymity order banning the publication of Mohamed's name has been lifted to allow police to make a public appeal, following an application from the Home Secretary.
The Security Minister, James Brokenshire, said: "National security is the Government's top priority and the police are doing everything in their power to apprehend this man as quickly as possible."
Somalia-born Mohamed, who is 5ft 8in tall and of medium build, arrived at the An-Noor Masjid and Community Centre in Church Road, Acton, at 10am on Friday and was last seen there at 3.15pm that day.
A spokesman for Scotland Yard said: "The Counter Terrorism Command immediately launched inquiries to trace Mr Mohamed and these continue.
"Ports and borders were notified with his photograph and details circulated nationally. Public safety remains our priority."
Tpims, which include restrictions on overnight residence, travel and finance, are imposed by judges who are given access to secret evidence that can not be placed before juries.
He is the second person to breach a Tpim since they were introduced to replace control orders early last year. In December, Ibrahim Magag, who is understood to have attended terror training camps in Somalia, absconded after ripping off his electronic tag. Police are still searching for him.
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