Scotland Yard has apologised for re-employing a riot policeman with a chequered disciplinary record after he was acquitted of killing Ian Tomlinson at the G20 protests in London in 2009.

The jury at Southwark crown court, who took four days to clear PC Simon Harwood of manslaughter on a majority verdict, was not told that the officer had been investigated a number of other times for alleged violence and misconduct.

Harwood quit the Metropolitan police on health grounds in 2001, shortly before a planned disciplinary hearing into claims that while off-duty he illegally tried to arrest a man in a road rage incident, altering notes retrospectively to justify his actions.

He was nonetheless able to join another force, Surrey, returning to the Met in 2005. In a string of other alleged incidents Harwood was accused of having punched, throttled, kneed or threatened other suspects while in uniform, although only one complaint was upheld.

The Independent Police Complaints Commission described the chain of events around Harwood's rejoining his old force before becoming part of its elite Territorial Support Group as "simply staggering".

During the trial the officer accepted he was wrong in retrospect to have hit Tomlinson on the back of the leg and shoved him to the pavement as the 47-year-old walked slowly away from police lines on the evening of 1 April 2009, but told an often emotional trial that he believed at the time the action had been necessary.

The trial hinged on two key questions: first, whether Harwood's actions amounted to a criminal assault; second, whether they directly led to Tomlinson's death. After three days of deliberation, the jury was unable to reach a unanimous verdict, suggesting they were unconvinced by one or both of these two key prosecution claims. Told by the judge they could return a majority 10 to two verdict, they found Harwood not guilty.

On Thursday a friend of Harwood's claimed that evidence from a trauma specialist, Alastair Wilson, who raised the hypothesis that analysis of Tomlinson's blood indicated his internal bleeding could have started before the fall, had been crucial to the acquittal.

"It was a vital piece of evidence that wasn't heard at the inquest. He was never guilty of manslaughter. He may have been guilty of assault for the baton strike, but he didn't kill him."

Tomlinson's widow and nine children and step-children gasped and walked out of the public gallery weeping when the verdict was announced.

Harwood, 45, dressed in a dark suit, cried quietly in the dock before being embraced by his wife, Helen.

The verdict brings to an end one of the most high-profile cases of alleged police misconduct in recent years. The IPCC initially declined to independently investigate Tomlinson's death.

But it reversed the decision after the Guardian released video footage, recorded by a US businessman, showing Tomlinson being struck with a baton and pushed to the ground by a police officer.

The case leaves in its wake something of a legal quandary: last year, after hearing similar evidence, an inquest jury ruled that Tomlinson was "unlawfully killed" by a police officer. They did so on the same standard of proof – beyond reasonable doubt. Neither jury was told about Harwood's chequered disciplinary background.

Tomlinson's family said they would now take civil action against the Met, while the force said that it would hold Harwood to account in a public disciplinary hearing.

The Met's deputy assistant commissioner, Maxine de Brunner, conceded that Harwood should never have been allowed to re-join the force.

"It is clear that insufficient recording and checks meant that detailed information regarding the officer's misconduct history was not shared at key points. We got that wrong. Since then there have been huge changes to vetting processes. Now all applicants, including officers applying to becoming police staff, as well those re-joining or transferring from other police services, are formally vetted and this involves a full misconduct intelligence check."

De Brunner did not explicitly apologise to Tomlinson's family, saying only that their "sympathies" were with them.

Outside the court, Tomlinson's stepson, Paul King, statementread a brief : "It really hurts. But this is not the end – we are not giving up on justice for Ian. There has to be one formal and final answer to the question: 'Who killed Ian?' And we will now pursue this in the civil court." He was too emotional to answer questions.

Harwood and his wife left soon afterwards, and gave no comment.

British juries are notoriously reluctant to convict police for serious alleged crimes carried out on duty. No police officer has been found guilty of manslaughter in 25 years, despite hundreds of cases in which families have alleged wrongdoing.

Deborah Coles, from the charity Inquest, called the verdict "a damning reflection of the systemic problems inherent in the current investigation system where deaths following police use of force are not treated as potential crimes".

Deborah Glass, deputy head of the IPCC, said significant questions remained about Harwood's actions on the day, to be examined at the disciplinary hearing.

She added: "There are also questions in this case that the [Met] must answer. PC Harwood was able to retire from the [force] while facing disciplinary proceedings for previous alleged misconduct towards a member of the public. That he was then re-employed by the force, first in a civilian role and later as a constable, is simply staggering and raises considerable concerns about their vetting procedures."

Separately, an IPCC report found its investigators had not been told about three police witnesses who, it was discovered 48 hours after Tomlinson's death, confirmed they saw a colleague strike the news seller with a baton and push him to the ground. The watchdog also confirmed that, in the days after his death, City of London police withheld details of his injuries from his family.

Tomlinson was going home from work near Monument tube station on the day he died, on Cornill, near the Bank of England. He had been an alcoholic for some years and was living in a hostel. His route home took him through the G20 rallies, where protesters had clashed with police. He had his back to officers and hands in his pockets when Harwood stepped forward, struck him with a baton and shoved him to the ground. Police did not go to his aid. Tomlinson rose to his feet and stumbled further down the road. He then collapsed and died.

Three forensic pathologists concluded that he died of internal bleeding from injuries sustained in the fall. They were contradicted by the pathologist Freddy Patel, the first to do an autopsy, who said Tomlinson died of heart failure.

Harwood presented himself after seeing videos of his encounter with Tomlinson on TV news. It later transpired that, prior to his encounter with Tomlinson, the police officer had become separated from his van. Minutes before the incident with the newspaper seller, Harwood had had several altercations with protesters and bystanders, including a BBC cameraman whom he pulled to the ground.

The director of public prosecutions, Keir Starmer, initially decided not to bring any charges against Harwood, citing contradictions created by Patel's evidence. However that decision was reversed 14 months ago, following the inquest verdict.