Image copyright Twitter Image caption Siddhartha Dhar is a Muslim convert who uses the name Abu Rumaysah

It is time to "blow away" all the "nonsense" that prevents police from properly pursuing extremist suspects, London Mayor Boris Johnson has said.

In a BBC interview, Mr Johnson was asked about Londoner Siddhartha Dhar, who is thought to have fronted a so-called Islamic State propaganda video after fleeing the UK while on bail.

He said laws governing bail were "pretty ineffective" and police work was hampered by "very uppity lawyers".

The UK needed a more "robust" approach.

I'm not saying 'stuff their human rights' because that's not the way we do things in this country. But let's be pretty robust in our approach Boris Johnson, Mayor of London

The UK's security authorities have faced mounting questions over how Mr Dhar was able to leave the UK in 2014 while on police bail, one of the conditions of which was handing over his passport.

Muslim convert Mr Dhar, also known as Abu Rumaysah, had been arrested on suspicion of encouraging terrorism, but later travelled to Syria.

"Police bail's a pretty ineffective system," Mr Johnson told BBC Radio London.

"Under the law, they have various restrictions under human rights as to what they can actually do.

"And the police are constantly monitored by very, very uppity lawyers... who are drawn to vindicate the rights of their clients."

He pointed out that 750 British nationals had gone to Syrian territories of Islamic State - which the government refers to as Daesh.

"When you've got somebody like this who's a member of... whatever organisation, and we've seen what's happening in the Daesh territories in Syria, let's not monkey around, let's make sure we follow them," he added.

'Special care'

"I'm not saying 'stuff their human rights' because that's not the way we do things in this country. But let's be pretty robust in our approach."

Former director of public prosecutions Lord Macdonald said he believed terror suspects should not be granted police bail unless the decision was signed off by a prosecutor.

"Given the sensitivity of these cases, and their obvious national security implications, any consideration of bail requires special care," he said.

"The Siddharta Dhar case shows that it is not sufficient or safe for the police to decide this question for themselves."

Nazir Afzal, who was chief crown prosecutor in north-west England from 2011 until last year, said there was a "strong case" for prosecutors to have the final say.

A CPS spokesman said it worked "very closely" with the police on all terrorism cases, including pre-arrest, but any decision on bail conditions before a charging decision was a matter for the police.

Image copyright IS video Image caption A still from the latest IS propaganda video

Earlier this week, official sources told the BBC that Mr Dhar, from Walthamstow in east London, was the focus of investigations into the video, which purported to show the killing of five men.

IS has said the men had been spying for the UK.

IS seized large swathes of territory in Syria and Iraq in 2014, when it formally declared the establishment of a "caliphate" - a state governed in accordance with Islamic law, or Sharia.

The group has previously released propaganda videos of killings, including footage showing the apparent beheading of two US journalists, James Foley and Steven Sotloff, and two British aid workers, David Haines and Alan Henning.