Pressure is mounting on the Government to explain how a terror suspect escaped surveillance by changing into a burka during a visit to a mosque.
Mohammed Ahmed Mohamed is understood to have received training and fought overseas for a l-Shabaab, the Somalia-based cell of the militant Islamist group al-Qaida.
The 27-year-old entered a west London mosque on Friday in Western-style clothes but CCTV images later showed him leaving with his face and body fully covered by the traditional Islamic garment.
Home Secretary Theresa May will make a statement on his disappearance to the House of Commons at 3.30pm.
He is subject to a Terrorism Prevention and Investigation Measures (Tpim) notice, which was imposed primarily to prevent overseas travel.
Mohamed is the second person to breach a Tpim since they were introduced to replace control orders in early 2012. Ibrahim Magag ripped off his electronic tag last December and vanished in a black cab.
Shadow home secretary Yvette Cooper described the situation as "extremely serious" and demanded more information from Home Secretary Theresa May on how Mohamed was able to abscond.
"Clearly police and security agencies will be doing everything possible to locate this terror suspect and ensure public safety," she said.
"The Home Secretary also needs to provide information about the decisions made over Mohammed Ahmed Mohamed's Tpim, how he was able to abscond and what the risks to the public are."
Ms Cooper added that Ms May needs to "provide rapid information about the extent and adequacy of the restrictions" on Mohamed.
Somalia-born Mohamed, who is 5ft 8in and of medium build, is "not considered at this time to represent a direct threat to the public", Scotland Yard said.
He was named after a court-imposed anonymity order was lifted by the Home Secretary to allow police to make a public appeal.
It is understood he took part in terrorist training in 2008 and is believed to have helped v arious individuals travel from the UK to Somalia to allow them to engage in terrorism-related activity.
Mohamed is also suspected of helping to plan attacks in Somalia and overseas, including an attack intended for the Juba Hotel in Mogadishu in August 2010.
He 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 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 cannot be placed before juries. They do not allow for the relocation of suspects, as control orders did.
Unlike control orders, Tpims have a maximum time limit of two years. Control orders could be extended year on year without limit, while Tpims can be extended after a year for another 12 months before they expire.
There were nine Tpims in force as of August 31, including eight against British suspects.
Mohamed's disappearance prompted David Anderson QC, the independent reviewer of terrorism legislation, to reveal criminal charges had been dropped on Friday against a number of Tpim subjects for allegedly tampering with electronic monitoring tags. It is not known whether Mohamed was among the subjects.
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.
Mr Anderson said in his first report on Tpims in March that the two-year limit was the "boldest" change from control orders made by the Government, adding that it was "tempting, in the most serious cases, to wish for longer".
Commenting after Mohamed's disappearance, Mr Anderson said prosecutions for Tpim breaches are "difficult".
He added: "Criminal charges against a number of Tpim subjects for allegedly breaching the terms of their Tpims by tampering with their GPS tags were dropped on Friday after no evidence was offered by the Crown."
Security Minister James Brokenshire defended the measures, describing them as providing a "robust mechanism" to manage suspects and reassure the public.
He 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."
Magag, a 28-year-old Somali, has not been seen since Boxing Day last year.
He is thought to be a member of a UK-based group of extremists who support the Al Shabaab terrorist organisation in east Africa.
Magag was made the subject of a stringent control order in 2009 but the restrictions expired when control orders were replaced by Tpims last year.
An-Noor Masjid and Community Centre said it did not intend to comment.
Prime Minister David Cameron's official spokesman said steps will be taken to see whether any lessons need to be learned from the case.
The spokesman told a regular Westminster media briefing: "There has been a breach in the way that the police have said. The priority is of course to locate the individual.
"Obviously, in all of these types of things, we look at whether there are lessons that can be learnt.
"But the reasons why the Government introduced the Tpim regime remain the same and the Prime Minister's view about Tpims haven't changed."
Mohamed helped support a UK-based network for terrorism-related activity in Somalia, which included Magag among its members, court documents have revealed.
The network had access to money, false passports and documentation, as well as equipment, with Mohamed being involved in fundraising for terrorism-related activity.
Between 2008 and late 2010, Mohamed is also understood to have procured weapons for terrorism uses.
Lord Carlile, the former reviewer of anti-terror laws, said: "We were assured by the Government that extra money would be spent on surveillance to ensure that exactly this kind of event did not occur.
"Yet the person concerned was able to walk in the front door of a mosque as a man and out through another door as a woman, on CCTV which was not seen, apparently, by the authorities.
"And here we are talking about Somalia-based terrorism which is having extremely damaging effects on people throughout the world and caused the death of British people, among many others, recently in a shopping centre in Kenya."
The Liberal Democrat peer told BBC Radio 4's World At One the inability to force suspects to relocate from their home areas under the Tpim regime was a major weakness compared with the control order system it replaced.
He said a "very powerful part of control orders was the power to order someone to relocate to an area where he would be distant from those who might want to help him escape".
The lack of that power was "always going to be a vulnerability in the way Tpims operate".