A Home Office minister has suggested the government might seek to strengthen control orders after it emerged two terror suspects had gone on the run.
Control orders are part of the anti-terrorism effort
The suspects, put under control orders to restrict their movements, include a British man who fled through a window of a mental health unit two weeks ago.
Tony McNulty said the measures had always been a "second best option" and were under review.
Civil liberties campaigners have called the orders "unfair" and "a farce".
A major police investigation is ongoing after the men's disappearance, and it is understood ports and airports have been notified.
The British suspect, who is of Pakistani descent, is accused by the authorities of wanting to go to Iraq to fight. He had been subject to a control order since March.
The second man, an Iraqi, is thought to have been missing for some months.
'Provision in law'
Mr McNulty hinted at the need for a stronger version of control orders which would depart from the European Convention on Human Rights.
He told BBC Two's Newsnight: "We'll keep this under review. We have provision in the law for a different form of control orders and at this stage we don't rule either in or out."
Asked why the Home Office had not told the public earlier about the suspects' escapes, he said: "I can say very clearly and assure people that the people who needed to know in both cases have known."
In response to suggestions the two suspects could carry out an attack at any time, Mr McNulty said: "On balance, I don't think that's the case at all."
Control orders were brought in for cases where people are suspected of being involved in terrorism-related activity, but there is not enough evidence for a criminal prosecution.
They were introduced last year after Law Lords ruled it was illegal for the government to hold terror suspects indefinitely in jail without charge or trial.
When placed under a control order, suspects can be tagged, confined to their homes and banned from communicating with others.
They are said to be reserved for dangerous terror suspects.
The family of the British suspect say they are concerned for his safety and are appealing for his return to fight the allegations against him.
The suspect maintains that he was arrested during a recent visit to Pakistan, held for seven months and tortured by the intelligence services.
His brother told BBC News: "We don't know what to think. We don't know what sort of mind he might be in."
The Iraqi suspect managed to get his curfew conditions relaxed earlier this year before disappearing.
Shami Chakrabarti, director of human rights group Liberty, said control orders did not work.
"This confirms our worst fears about the farce that is the control orders regime. They are both unsafe and fundamentally unfair," she said.
"If someone is truly a dangerous terror suspect, why would you leave them at large?
"On the other hand it is completely cruel and unfair to label someone a terrorist and to subject them to a range of punishments for years on end without ever charging them or putting them on trial."
Shadow Home Secretary David Davis described the case of the suspect missing from the mental health unit as "extraordinary".
He added that it was "hard to understand how this man was allowed to escape, especially while undergoing psychiatric assessment".
Nick Clegg, for the Liberal Democrats, said the case was a "huge embarrassment" for ministers.
He said: "The danger of control orders is that they short-circuit due process and keep suspects in a state of limbo."