Terror suspect Mohammed Ahmed Mohamed is being hunted by counter-terrorism officers from Scotland Yard after breaching his terrorism prevention and investigation measures notice.
The 27-year-old was last seen on Friday afternoon at a mosque in Acton, west London.
The force said that Mohamed is not considered to be a direct threat to the public, but advised anyone who sees him not to approach him and to call 999 instead.
Somalia-born Mohamed was wearing traditional Islamic clothing when he disappeared, having changed into it at the An-Noor Masjid and Community Centre in Church Road, Acton, Scotland Yard said.
He had arrived at the mosque at 10am on Friday and was last seen there at 3.15pm that day.
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."
A court-imposed anonymity order banning the publication of Mohamed's name was lifted yesterday to allow police to make a public appeal for information.
He is 5ft 8in tall and is of medium build. Anyone with information about him should call confidential Anti-Terrorist Hotline on 0800 789 321.
CCTV images issued by Scotland Yard show the suspect wearing a burka after leaving the mosque, with his face totally covered.
He had arrived at the building earlier in Western-style clothing.