Judge Mark Lucraft QC, the Chief Coroner of England and Wales, said the Metropolitan Police had allowed the entrance PC Keith Palmer was guarding to be neglected by “roving” armed officers.
PC Palmer's widow said he was "left to die", adding: "How could Keith have been left alone, unarmed, guarding an open gate at one of the most iconic buildings in the world and one of the country’s top terrorist targets?”
Judge Lucraft is to draw up a “prevention of future deaths report” that will contain recommendations for the Metropolitan Police and parliamentary security officials.
Please allow a moment for the live blog to load.
Masood, 52, drove into pedestrians on Westminster Bridge on March 22 last year, killing American tourist Kurt Cochran, 54, retired window cleaner Leslie Rhodes, 75, Aysha Frade, 44, and Romanian tourist Andreea Cristea, 31.
Their relatives expressed hope that "lessons had been learned" from the gruelling three-week inquests.
Delivering his conclusions at the Old Bailey, Judge Lucraft said the two armed officers assigned to the area had not been near Carriage Gates for almost an hour before Khalid Masood struck because they were unaware of official guidance.
“They had been some distance away and out of view of the entrance because they had understood their duty to involve a roving patrol around the yard,” Judge Lucraft said.
“Due to shortcomings in the security system at New Palace Yard, including the supervision of those engaged in such duties, the armed officers were not aware of a requirement to remain in close proximity to the gates.
“Had they been stationed there, it is possible that they may have been able to prevent PC Palmer suffering fatal injuries.”
PC Palmer's widow said her husband had been “left to die” by security failings and accused authorities of letting “lax security” continue at the Houses of Parliament for years.
In his closing argument, Gareth Patterson QC, for families of victims on the bridge, urged the coroner to make a report on the circumstances of the case to “protect the public”.
On the role of MI5, whose knowledge of Masood has been heavily scrutinised, he said: “We do think there is room for improvement in terms of decision-making.”
He suggested security services should look again at when to investigate suspects and when to stop, and to take account of violent backgrounds.
Mr Patterson also called for the Government to “try again” with tightening rules for hiring cars.
Dominic Adamson, representing PC Palmer's widow Michelle, said that there was “a systematic failure” in protecting unarmed officers on guard at the Palace of Westminster.
Good morning and welcome to our live coverage from the conclusion of the Westminster attack inquests.
Mark Lucraft QC, the chief coroner of England and Wales, is due to give his conclusions at the Old Bailey on the deaths of four civilians and police officer Keith Palmer, who were murdered by terrorist Khalid Masood.
He will consider whether factors including security at the Houses of Parliament, the lack of barriers on Westminster Bridge, police body armour and security service investigations into Masood played a role in their deaths.
The court will hear further closing submissions in the morning, before the coroner begins giving his conclusions.
Representatives of the victims' families started making submissions to the coroner yesterday. They urged the coroner to make a report on the circumstances of the case to “protect the public” and called for changes to the way MI5 starts and stops investigations into potential terrorists.
A lawyer representing PC Palmer's wife said there was “a systematic failure” in protecting unarmed officers on guard at the Palace of Westminster.
The inquests are currently hearing closing submissions from interested parties, after which the coroner will start delivering his conclusions.
Hugo Keith, a lawyer representing the Metropolitan Police, has been arguing against claims by the family of PC Keith Palmer that having armed officers stationed at Carriage Gates could have saved his life
Citing evidence given by firearms officers, he said that even if they had been on Carriage Gates at the time of the attack, they would have run towards the site where Khalid Masood crashed his car and left it unguarded.
The noise made by the vehicle crashing into a low wall and railings surrounding the Houses of Parliament sparked initial reports of an explosion, and officers at Parliament were unaware that victims had been run over on Westminster Bridge.
Mr Keith argued that even if armed officers remained at Carriage Gates, they may not have been able to take a safe shot at Masood as he ran through members of the public at speed.
He says it is "not arguable that there was a breach of article 2" of the European Convention on Human Rights,
The article covers the “right to life” and where it is triggered following a death, a broader inquiry into the death occurs that specifically considers state and statutory negligence
Mr Keith argues that the fact the Westminster inquests have not been designated under article 2 "had made no difference to the format, scope and rigorousness" of the process
But counsel to the inquest Jonathan Hough QC does not accept his argument. He says that Carriage Gates had been identified as a weak point in the parliamentary perimiter by security reviews, and that the estate contains "thousands of people...including very important figures who make an attractive target for terrorists"
He says the systems for police changing post were "arguably deficient", leaving officers unaware of their instructions. "We also see that the system could be improved"
Mr Hough says: "It cannot be that it is unarguable that there was a means of ensuring that almost all of the time officers in this critical place were complying with their instructions."
He says the question is "whether it is arguable that a systemic breach deprived PC Palmer of a real and substantial chance of survival"