The intelligence agencies may have made a "slip up" in failing to impose tighter controls on Jihadi John due to the sheer number of suspects they have to cope with, the counter-terrorism laws watchdog said.

David Anderson QC said he was not surprised that Mohammed Emwazi, who has been named as the masked militant seen in Islamic State videos, was not subject to measures such as house arrest despite being on MI5's radar because the UK's spies and counter-terrorism police were "very busy".

University of Westminster graduate Emwazi was known to the Security Service for some time before leaving for Syria in 2013 and beginning on the road to international notoriety.

Mr Anderson, the independent reviewer of terror legislation, said "perhaps they did slip up in this case" but it was "difficult not to have sympathy" with the agencies because of the scale of the challenge they face.

Asked if he was surprised that MI5 had not prevented Emwazi from leaving the country he told BBC Radio 4's World at One: "I'm afraid it's not surprising to me at all, because I do spend time with the intelligence agencies and with the police. They are very busy at the moment and they were very busy then.

"There are a lot of people that they are a bit suspicious of, there are hundreds, probably thousands.

"You can't round all those people up without a trial and declare that they are subject to some form of house arrest.

"You need, at the very least, to be sure that they have participated in terrorism before you start taking measures like that.

"Now I don't know, perhaps they did slip up in this case but one won't know until there's been an inquiry or a report of some kind.

"But it's difficult not to have sympathy with them when you see just how many cases they have to look at, how their job is really all about priorities.

"A lot of people talk a good game when it comes to terrorism, the knack is to try and identify the few who are going to do something about it."

London Mayor Boris Johnson said the Government had made a mistake to replace the control orders regime, which allowed the relocation of terror suspects, with the less-restrictive Terrorism Prevention and Investigation Measures (Tpims).

He told the BBC: "I do think that was a mistake, we are now back on the right track.

"The politicians who made that mistake need to think very carefully about why they did it and I think the benefit of the doubt was, I'm afraid, given too much to those who wish us serious harm."

But Mr Anderson said it was a "false debate" because the full control order regime was available until the end of 2011 and some of those associated with Emwazi had been subject to them.

He said: "There is a political debate here, but I don't think this is a very helpful case to have it on because control orders in their previous, unadulterated, form were available until the end of 2011.

"He in fact had a couple of associates, according to evidence given to the High Court, who were under control orders - one of them Ibrahim Magag, the chap who when he was brought back to London absconded at the end of 2012.

"So in a sense it's a false debate if you are looking at the period before 2012."