Britain's most senior counter-terrorism officer has described police bail as "weak" and "toothless" as he revealed officers are in talks with the Government about introducing tougher powers.
Metropolitan Police assistant commissioner Mark Rowley disclosed that discussions are being held with the Home Office about strengthening the arrangements.
He told MPs that more than a hundred terror suspects are currently on bail as investigations continue.
A furore erupted following revelations about lapses that allowed Islamic State (IS) video suspect Abu Rumaysah to leave the UK despite being on police bail.
Mr Rowley said: "Police bail is a weak provision and it is something that we are in conversation with the Home Office about whether there are ways to strengthen it."
The officer told the Commons Home Affairs Committee the only breach of police bail which is a criminal offence is failure to return on the required date.
He said: "If conditions are put on somebody which may be a curfew, may be non-association, may be surrendering passport immediately or in a short period of time, if that person breaches those conditions in a police bail process, we can arrest them, but we can't prosecute them.
"They are back in police custody with a very tight timescale so they just get bailed again with the same conditions. It is fairly toothless."
Mr Rowley called for passport collection to be made a condition of police bail and for breaches to be made a criminal offence.
Counter-terrorism units are making an unprecedented number of arrests.
Mr Rowley said that of 339 arrests last year, around a third are ongoing investigations in which suspects are on bail.
"The control around that third would be stronger, if police bail was a stronger provision," he said.
Rumaysah - born a Hindu called Siddhartha Dhar - was able to leave the country in 2014 after being arrested six times.
He departed with his family the day after being released on bail, travelling to Paris and then Syria.
The episode has come under the spotlight following unconfirmed claims that Rumaysah was the masked militant at the centre of a film released earlier this month showing the murder of five men accused by IS of spying for the UK.
Reports claim other suspected extremists who were either on bail or subject to travel restrictions have left Britain in the past 18 months.
Mr Rowley, who did not discuss the Dhar case specifically, said there are thousands of individuals of concern.
"We are having to prioritise against the highest risk, which means there will be some people on bail who won't be the highest risk individuals because we are focused on those more intent on attack planning quite often," he said.
Appearing before the Commons Liaison Committee, David Cameron said he would consider Mr Rowley's call for a change in the law.
"I am very happy to look carefully at that. Frankly, there is more that we need to do here," he said.
On the Dhar episode, the Prime Minister said: "I think in this individual case the passport wasn't on him when he was arrested and he was already at the limit of how long he could be detained under the Pace (Police and Criminal Evidence Act) powers, and so the police couldn't legally hold him while his passport was sought.
"His home address was searched and had the passport been found it could have been seized, but it wasn't found."