A Somali terror suspect has given the authorities the slip by dressing in a burka.
A nationwide hunt is underway for Mohammed Ahmed Mohamed, who walked into a mosque wearing Western clothes, but left with his face and body covered.
His escape is highly embarrassing for the government, and ports and borders are on alert in case he tries to flee the country.
Mohamed, 27, has connections with Al Shabab, the terrorist group behind the recent Kenyan shopping centre massacre.
He is subject to a Terrorism Prevention and Investigation Measures notice (Tpim).
Scotland Yard said he was "not considered at this time to represent a direct threat to the public", but urged anyone who sees him to call 999 immediately.
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.
"The police and security services do not believe that this man poses a direct threat to the public in the UK."
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.
He is the second person to breach a Tpim since they were introduced to replace control orders in early 2012.
A Scotland Yard spokesman 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 the Home Secretary who is given access to secret evidence that can not be placed before juries. They do not allow for the relocation of suspects, as control orders did.
Last December, Ibrahim Magag, who is understood to have attended terrorist training camps in Somalia, absconded from a Tpim notice after ripping off his electronic tag. The police search for him is continuing.
The terrorism watchdog warned earlier this year that Tpims could allow those deemed potentially dangerous to be left "free and unconstrained" in the absence of prosecution or new evidence of terrorism-related activity.
Shadow home secretary Yvette Cooper described the situation as "extremely serious" and called for an urgent independent investigation.