A fourth victim has died after a terrorist drove his car into dozens of people on Westminster Bridge before crashing it into the gates of Parliament.
Khalid Masood was shot dead by police moments after fatally stabbing PC Keith Palmer on Wednesday and mowing down people on the bridge.
Police said a 75-year-old man died in hospital from his injuries on Thursday evening.
A Scotland Yard spokesman said: “Detectives investigating the terrorist attack in Westminster on Wednesday, 22 March can confirm that a 75-year-old man died tonight, Thursday, 23 March.
“The man had been receiving medical treatment in hospital following the attack and life support was withdrawn this evening.
“Next of kin have been informed and are receiving support from specially trained family liaison officers."
The man has not yet been named.
Two people - American Kurt Cochran and Spaniard Aysha Frade - were also killed and about 40 others hurt on the bridge.
Mr Cochran and his wife, Melissa, on the last day of a trip celebrating their 25th anniversary, were visiting her parents, who are serving as Mormon missionaries in London. Mrs Cochran was badly injured.
Ms Frade, who worked in administration at independent sixth-form school DLD College London, in Westminster, is understood to have been 43 and married with two daughters.
Isis has claimed responsibility for the attack.
The news of the pensioner's death came just hours after Londoner's held an emotional vigil for the victims in Trafalgar Square.
Earlier, police had revealed the killer's identity as Masood, a 52-year-old British-born man who had previously been investigated by MI5 for extremism.
He had a string of convictions to his name dating back to 1983 but had never been convicted of terrorism, police said.
Masood was born in Kent but had recently moved to Birmingham. Neighbours claimed he was a married English teacher with three children.
Police have also arrested three women and five men on suspicion of preparing terrorist acts in connection with the investigation.
The arrests followed raids on addresses in Brighton, London and Birmingham.
Detectives were continuing to search properties in Carmarthenshire, east London and Birmingham on Thursday.
Scotland Yard said Masood was not the subject of any current investigations before Wednesday's outrage and there was "no prior intelligence about his intent to mount a terrorist attack".
However, he was known to police and MI5 and had convictions for assaults, including GBH, possession of offensive weapons and public order offences.
Masood was born in Kent on Christmas Day in 1964 and was also known by a number of aliases.
One former neighbour, Iwona Romek, speaking in Winson Green, Birmingham, said she could not believe her eyes when she realised the man who had lived near her was the man responsible for the attack.
She said: "Now I'm scared that someone like that was living close to me."
In Trafalgar Square, huge crowds gathered for the candlelit vigil in memory of the victims on Thursday evening.
They had responded to a personal invitation by London mayor Sadiq Khan to come together in solidarity and sympathy.
In praise of the bravery of the emergency services responding to the attack, the mayor said: "When Londoners face adversity we always pull together.
"We stand up for our values and we show the world we are the greatest city in the world."
Earlier, Prime Minister Theresa May disclosed the terrorist was British-born and was once investigated in relation to concerns about violent extremism some years ago, but was a "peripheral figure".
Home Secretary Amber Rudd defended the security and intelligence agencies, saying: "The fact that he was known to them doesn't mean that somebody has 24-hour cover."
She disclosed Masood had spent time in jail, but said it was not for terrorist-related offences.
The Prime Minister addressed MPs as they gathered at the usual time inside the Palace of Westminster.
In a defiant message to a packed House of Commons, Mrs May said: "We will never waver in the face of terrorism."
Paying tribute to Pc Palmer, she said: "He was every inch a hero and his actions will never be forgotten."
The officer's family described him as "brave and courageous", saying his friends and relatives are "shocked and devastated".
Meanwhile, Islamic State made its first public pronouncement since the atrocity, claiming in a statement: "The attacker yesterday in front of the British Parliament in London was a soldier of the Islamic State executing the operation in response to calls to target citizens of coalition nations."
Commentators pointed out the terror group has a record of opportunistically claiming attacks and said it was significant the statement did not appear to claim it had directed the strike.
A minute's silence was held nationwide at 9.33am, including in the Palace of Westminster and at New Scotland Yard.
Up to 40 other people were also injured in the attack, with casualties including Britons, French children, Romanians, South Koreans, Greeks, and people from Germany, Poland, Ireland, China, Italy and the United States.
Three police officers were also hurt, two of them seriously.
In a sign of Westminster's determination to return to business as usual, the entrance to Parliament which saw the deaths of Pc Palmer and Masood, was back in use.
Parliamentary passholders were able to use the Carriage Gates, walking within metres of the spot where Pc Palmer fell as he confronted Masood, although armed police were in attendance at the entrance.
Roads around the Palace of Westminster, including Parliament Square, were reopened and soon busy with traffic.