Khalid Masood, 52, was shot dead by two personal protection officers after he had mown down four people on the bridge and then stabbed PC Palmer, who was stationed at the gates to the Palace of Westminster in March last year.
"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," the coroner, Judge Mark Lucraft QC, said on Wednesday, October 3.
"Had they been stationed there it is possible that they may have been able to prevent PC Palmer suffering fatal injuries," added Judge Lucraft, who recorded a verdict of unlawful killing on all five victims.
In
Armed Police Blunders
The inquest heard two armed officers ran towards the area where Masood crashed his car but he had already got out and ran around the corner and entered the gates of Parliament, where he was confronted by PC Palmer.
PC Palmer's widow, Michelle, told reporters after the inquest they felt he had been left in a vulnerable situation when armed officers wandered away from the gate to patrol the building.
"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? He was left at a vulnerable location with no protection to die," said Mrs. Palmer.
PC Palmer challenged Masood, an Islamic convert who was born Adrian Elms, but was stabbed with two knives in a "ferocious" attack.
Masood was known to the authorities and had come under the radar of MI5 several years ago as a "peripheral" figure in a terrorism investigation, Prime Minister Theresa May told MPs a few days after the attack.
He is believed to have been a "lone wolf" attacker who had been radicalized online.
A separate inquest into the death of Masood himself is due to begin on Thursday, October 4.
Police Chief Says Sorry For PC Palmer's Death
In a statement, Assistant Commissioner Neil Basu said the Metropolitan Police (MPS) unreservedly accepted the coroner's conclusions.
"The Chief Coroner said: 'due to the 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 the 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," said Mr. Basu.
"Even the possibility that the MPS lost the chance to prevent the murder of a brave and courageous officer is unacceptable. For the loss of that possibility to protect him from Khalid Masood, we are deeply sorry," said Mr. Basu.
"Security arrangements have been substantially changed since the terrible events of 22 March 2017. However, everything that we can possibly do to improve the position further, and we await the Chief Coroner's detailed conclusions, will be done," he added.
PC Palmer's friend and boss, Chief Inspector Neil Sawyer, said the dead man loved his job at the Palace of Westminster.
'The Ultimate Sacrifice'
"In 2017 he made the ultimate sacrifice in defense of Parliament. It didn't surprise me when I heard. He would never back away. He will rightly always be remembers for his brave actions but he was also our dear friend," Chief Insp. Sawyer told the inquest.
The inquest, sitting at the Old Bailey, was shown still images from CCTV cameras of the terrifying moments when Masood mounted the pavement in a rented Hyundai Tucson and struck innocent tourists and Londoners as they went about their business.
Retired window cleaner Les Rhodes, 75, was dragged 33 meters under the wheels of Masood's vehicle and died in hospital the following day.
Kurt Cochran, 54, from Utah in the United States, who was celebrating his wedding anniversary with his wife Melissa, was hurled over the parapet and suffered fatal head injuries.
Aysha Frade, 43, a British mother-of-two, was struck from behind and ended up under the wheels of a bus, suffering "catastrophic" injuries and dying instantly.
Andreea Cristea, 31, an interior designer from Romania, who was visiting London with her boyfriend, was flicked into the river Thames and was fished out eight minutes later.