The widow of the police officer killed during the Westminster attack says her husband was left "in a vulnerable location, with no protection, to die".
Westminster terror attack: 'How could Keith have been left alone?'
It comes after the coroner said Keith Palmer's death on 22 March last year could "possibly" have been prevented if armed officers had been nearby.
"Shortcomings in the security system" meant they were unaware they should have been on guard near his position.
All five victims of the terror attack were "unlawfully killed", he ruled.
They were PC Palmer, 48, Kurt Cochran, 54, Leslie Rhodes, 75, Aysha Frade, 44, and Andreea Cristea, 31.
Their killer, Khalid Masood, was shot dead by police just inside the entrance to the Palace of Westminster. The inquest into his death will take place next.
What did the coroner say?
Chief Coroner Mark Lucraft QC, at the Old Bailey, has carried out the month-long inquests of those who died.
Giving his ruling on Wednesday, he praised the "great dignity" of their families throughout the process and also the "quite overwhelming" response from medics and members of the public.
He said that on on 22 March 2017, Masood had driven a hired SUV at an average speed of 30 to 42mph with "clear murderous intent", killing four pedestrians and seriously injuring 29 others, before attacking PC Palmer.
The coroner highlighted the actions of one victim, Mr Cochran, whom he said had "pushed his wife away from the path of Masood's vehicle" - "an instinctive act" which "almost certainly had the effect" of saving her life.
As part of the inquest, the coroner has also been examining security issues, including armed policing at Parliament, security on the bridge and what security service MI5 knew about Masood prior to his attack.
He said PC Palmer had been "fulfilling his job to protect the Palace and those within it" and "did not shrink from" performing that duty.
He was wearing a protective vest but was not armed, and the coroner said that had armed officers been stationed alongside him, they may have been able to prevent him suffering fatal injuries.
How have relatives reacted?
PC Palmer's widow, Michelle Palmer, said in a statement that the authorities let her husband down by failing to protect him and let the family down by failing to investigate his death properly.
She said: "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?"
She added: "What makes it even worse is that this lax security had been carrying on for years and it has taken what happened to Keith for things to change."
The parents and sisters of PC Palmer, who would have turned 50 on Wednesday, said the inquest had been "traumatic" and "extremely difficult".
They said the coroner should have gone further in his findings, accusing senior Met officers of closing ranks.
What's the police's response?
Neil Basu, head of counter-terror policing at Scotland Yard, said the Met accepted the coroner's conclusions "unreservedly".
He said: "Even the possibility that the Met lost the chance to prevent the murder of such a brave and courageous officer is unacceptable. For the loss of that possibility… we are deeply sorry."
Security arrangements had been "substantially changed" since that day, he added, and further changes would be made if recommended by the coroner.
The attack lasted82 seconds.
- 14:40:08- Masood's car mounts pavement on Westminster Bridge and hits pedestrians
- 14:40:18- It hits another group of pedestrians and the impact sends Andreea Cristea over the side and into the river
- 14:40:38- Masood crashes into perimeter fence of Palace of Westminster
- 14:40:59- First 999 call made to Met police
- 14:41:30- Masood leaves vehicle, stabbed PC Palmer, and is shot dead
Who were the victims?
Image copyrightMET POLICE/SOCIAL MEDIAImage captionClockwise from top left: PC Keith Palmer, Aysha Frade, Leslie Rhodes, Andreea Cristea and Kurt Cochran all lost their lives
PC Keith Palmerhad a young daughter. He was a member of the parliamentary and diplomatic protection squad who had served with the Met for 15 years.
Aysha Fradehad been walking across Westminster Bridge to pick up her two young daughters from school when she was killed. The British national worked as a PA at a nearby independent sixth-form.
Leslie Rhodesa retired window cleaner from Clapham, south London, had been visiting St Thomas' Hospital when he was hit by the car driven by Masood.
Kurt Cochranfrom the US state of Utah, was in London as part of a holiday celebrating 25 years of marriage to Melissa, who was seriously injured in the attack.
Andreea Cristeawas a Romanian tourist. She had been with her boyfriend, Andrei Burnaz, who had been planning to propose to her that day.