The government should review how public spaces are assessed as possible terrorism targets in the wake of the London Bridge attack, the chief coroner for England and Wales has said.
Eight people were killed in 2017 when three men drove into pedestrians on the bridge before stabbing others nearby.
Judge Mark Lucraft said the system for triggering extra security measures was "too rigid".
Families of the victims welcomed his "extensive" report.
The chief coroner's report into preventing future deaths also called on the government to consider criminalising "possession of the most serious material glorifying or encouraging terrorism".
He said the current law meant it might be impossible for police or MI5 to act even when "the material is of the most offensive and shocking character".
Judge Lucraft made 18 recommendations in total.
The attack began when Khuram Butt and two other men drove a rented van across London Bridge, striking pedestrians and killing the first two victims, Xavier Thomas and Christine Archibald.
There were no barriers on the bridge preventing the van from mounting the pavement, despite some concerns among experts that it could become a target.
After the attack, barriers were installed.
In his formal report, Judge Lucraft said: "The national criteria for identifying sites which would receive proactive advice were apparently too rigid."
He also said the home secretary needed some form of regular review of the list of vulnerable sites.
Public authorities such as councils may also need to be put under a legal duty to review open and crowded spaces to minimise the risks of a fatal attack.
The hypothetical new law that would criminalise possession of terrorism material could make it illegal to have a copy of the video that was live-streamed by the far-right killer who shot dead 51 worshippers at a mosque in Christchurch, New Zealand.
But other examples may be more difficult to explicitly list - not least because the government has struggled and failed to come up with a clear legal definition of extremism.
Would a law be able to outlaw the mere watching of a sermon from a terror group leader, even if the lecture does not in itself encourage violence?
The chief coroner says the answer may lie in how Parliament has successfully banned the possession of extreme pornography that depicts violence and abuse.
In that case, the law carefully sets out the harm MPs want to prevent. The question is whether a similar exercise could target supporters of terrorism without undermining legitimate free speech and inquiry.
The chief coroner also concluded that there was a significant gap in the law - highlighted by the fact that the ringleader Butt had a vast amount of extremist material on his phone but had not been charged with a crime.
"While there are offences of possessing a document likely to be useful to a person in committing an act of terrorism, and of disseminating terrorist publications, there is no offence of possessing terrorist or extremist propaganda material," he said.
The Old Bailey inquest heard that, in the months and years before the attack, Butt had viewed propaganda for the Islamic State group, violent images and sermons from extremist preachers.
Judge Lucraft suggested new laws could be introduced to tackle the prevalence of extremist material in the same way legislation has criminalised the most offensive pornography.
The inquests heard that Butt had been investigated by MI5 as a "subject of interest" - meaning he was one of the 3,000 suspects the security service was most concerned about.
The security service had been investigating Butt since 2015, but suspended its investigation on two occasions because of more pressing priorities. The latter suspension, for six weeks, concluded just a month before the attack.
"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," concluded Judge Lucraft.
He recommended that security services should consider whether to scale back rather than suspend work.
He said the emergency services should be more flexible during a marauding terrorist attack about how they allow paramedics and ambulance staff into zones considered to be dangerous - and that some police officers should receive advanced medical training analogous to "battlefield medicine".
He also called on ministers and industry to consider a means of instantly reporting the rental of a vehicle to the security services so that it can be checked against known suspects.
'Chaos and killings: 10 minutes at London Bridge'
It's 10.07pm and the sun has gone down on a warm summer's night in London.
Helen Boniface, from solicitors 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."
The home secretary must respond to the report by 10 January 2020, setting out details of action taken or proposed, or why no action will be taken.