Significant questions remain about the actions of Simon Harwood, the police watchdog has said.
Constable Harwood, 45, was acquitted yesterday at Southwark Crown Court of the manslaughter of Mr Tomlinson in the City of London in April 2009.
A jury of seven women and five men took 18 hours and 45 minutes to clear him.
He cried in the dock and his wife Helen sobbed as the verdicts were given, before they tearfully embraced as he was freed.
Mr Tomlinson's family also broke down, and outside the court his stepson, Paul King, called the verdict "a joke", vowing to pursue the issue in the civil courts.
Constable Harwood hit 47-year-old Mr Tomlinson with a baton and pushed him to the ground. The father-of-nine walked 75 yards before he collapsed. He died in hospital from internal injuries.
It can now be reported Constable Harwood faced a string of allegations about his behaviour before he came across Mr Tomlinson.
He retired on medical grounds from the Met while facing disciplinary proceedings for an alleged road rage incident – and despite this was later re-employed by the force as a police officer specialising in public order.
Last night Deborah Glass, deputy chairwoman of the Independent Police Complaints Commission, said the revelation was "staggering" .
She said: "It is clear that significant questions remain in connection with his actions on the day Ian Tomlinson died.
"Whether or not those actions were reasonable will be tested further at a misconduct hearing in September, which I have directed will be held in public.
"There are also questions in this case that the Metropolitan Police Service must answer.
"Constable Harwood was able to retire from the Metropolitan Police 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."
Constable Harwood, from Carshalton, Surrey, said he used reasonable force when he hit Mr Tomlinson with a baton and shoved him to the ground as he walked away from police lines.
Mr Tomlinson was an alcoholic and had slept rough for a number of years. An inquest was held into his death last year, where the jury returned a verdict of unlawful killing.
Jules Carey, solicitor for the Tomlinson family, said: "This is one of the hardest days for the family, and there have been many. Constable Harwood may have been acquitted of manslaughter by this jury, but another jury, at the inquest a year ago, found Ian Tomlinson had been unlawfully killed.
"It is impossible for this family to understand these two, apparently contradictory, verdicts. The family will now look to the civil courts to reconcile these verdicts, deliver justice and formally answer the question 'who killed Ian Tomlinson?"'
Metropolitan Police Deputy Assistant Commissioner Maxine de Brunner said the force had done all it could to co-operate with the investigation.
She said: "The evidence has now been put before the jury and they have reached their decision.
"It has always been the intention of the Metropolitan Police Service to hold the misconduct hearing as soon as possible. This was delayed pending the outcome of the trial.
"We will now need to review the misconduct case against Constable Harwood, as required by the Home Office guidance."
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