Mark Lucraft QC said current legislation means it may be impossible for police or MI5 to act against fanatics even when ‘the material is of the most offensive and shocking character’.

Eight people were killed and 48 others seriously injured when terrorists used a hired van to plough into pedestrians on London Bridge before attacking people at random around Borough Market on 3 June 2017.

Khuram Butt, 27, Rachid Redouane, 30, and Youssef Zaghba, 22, were lawfully killed after they were shot dead by armed police, an inquest jury found.

A separate inquest concluded that Xavier Thomas, 45, Chrissy Archibald, 30, Sara Zelenak, 21, James McMullan, 32, Kirsty Boden, 28, Alexandre Pigeard, 26, Sebastien Belanger, 36, and Ignacio Echeverria, 39, were unlawfully killed.

Families of the victims have spoken of ‘missed opportunities’ to prevent the atrocity and criticised the ‘slow response’ of the London Ambulance Service, which meant others had to step in to give aid to casualties.

The coroner highlighted 18 ‘matters of concern’ in a report published on Friday, which said ‘there is a risk that future deaths could occur unless action is taken’.

Butt, who was a subject of interest (SOI) in an active MI5 investigation at the time of the attack, had looked at extremist material online in the months and years leading up to it, including propaganda for so-called Islamic State, violent images and sermons from extremist preachers.

The coroner suggested new laws could be introduced to tackle extremist material in the same way legislation has criminalised the most offensive pornography.

He said: ‘While there are offences of possessing a document likely to be useful to a person in committing an act of terrorism (Section 58, Terrorism Act 2000), and of disseminating terrorist publications (Section 2, Terrorism Act 2006), there is no offence of possessing terrorist or extremist propaganda material.

‘It may be impossible to take action even when the material is of the most offensive and shocking character.

‘The evidence at the inquests indicates to me that the lack of such an offence may sometimes prevent counter-terror police taking disruptive action which could be valuable in their work of combating terrorism.

‘I have formed the view that consideration ought to be given to legislating for further offences of possession of the most serious material glorifying or encouraging terrorism.’

The inquest heard that police and MI5 did not recognise the threat posed by ringleader Butt, despite his association with Islamic State fanatic Anjem Choudary and an appearance in the documentary The Jihadi Next Door.

It emerged that the investigation was twice suspended due to pressure on resources and the authorities did not pass on tip-offs about his extremism, including one from a family member.

There was also a two-month delay in translating a request from the Italian authorities for information about Zaghba.

Mr Lucraft said: ‘Although MI5 must be able to prioritise and divert resources at times of greatest demand, the suspension of priority investigations is a matter of legitimate public concern.’

But he said it would be wrong to criticise the pre-attack investigation, calling the work of MI5 and police ‘generally thorough and rigorous’.

He made a series of suggestions to police and security chiefs about how they could work more closely and ensure that information from the public and foreign services are properly shared and communicated.

The attack came just three months after 52-year-old Khalid Masood mowed down pedestrians on Westminster Bridge with a hired car before stabbing police officer Keith Palmer to death.

And questions were raised about why there were no security measures in place to prevent vehicles being used as weapons on London Bridge, with barriers only installed in the immediate aftermath of the attack.

The coroner identified ‘weaknesses’ in the systems for putting such measures in place and suggested authorities should take steps to address the concerns.

He also said action should be taken to reduce the risk of rented vehicles being used in terror attacks, which could include automated checking of rentals against lists of SOIs.

Helen Boniface, a senior associate at law firm Hogan Lovells, who represented six families in the inquest, said: ‘We are pleased the Chief Coroner has recognised the risks presented by hateful extremism and terrorist propaganda, possession of which must be taken seriously, and the ease by which large vehicles may be hired by terrorist suspects.

‘The response on the night by many was commendable, especially members of the public who stayed to assist.

‘But failings and delays were also seen and the coroner identifies this through his report.

‘The emergency medical response to those who died in the Boro Bistro area remains disappointing to our clients, with no London Ambulance Service personnel entering this area until many hours after the attack.’

Those responsible for concerns raised in the report, which include Home Secretary Priti Patel, have until January 10 to respond, setting out details of action taken or proposed, or why no action will be taken.