The policeman found not guilty of killing Ian Tomlinson during the G20 riots had been previously accused of excessive force, ITV News has learned.
Pc Simon Harwood was also investigated for alleged off-duty road rage - but jurors who cleared him today were not told of his disciplinary record.
Trial judge Mr Justice Fulford rejected an application by prosecutors to admit the claims as evidence of the officer's 'bad character' because they were never proved.
Pc Harwood still faces the sack from the Metropolitan Police after admitting he was wrong to hit Mr Tomlinson and push him in the back.
But senior officers privately admit its likely he should not have been employed by the force at the time of the clash in the City of London in April 2009.
In 2001, Harwood was investigated for unlawful arrest, abuse of authority and discreditable conduct after confronting a motorist while off duty.
Those disciplinary proceedings were dropped after he retired on medical grounds.
Harwood then rejoined the Met as a civilian working as a despatch call handler.
In 2003, he applied to join Surrey police as an officer and after passing a medical was able to return to the rank and file.
Harwood did declare his employment history to Surrey but the force only received 'a few lines ' about the road rage claims from the Met.
While at Surrey, he was accused of excessive force when arresting a suspect for theft but the claim was unsubstantiated and his appraisals were graded 'satisfactory'.
A year later he transfered back to the Met and though the outstanding road rage complaint was identified original disciplinary proceedings were not resumed.
Deborah Glass, of the Independent Police Complaints Commission, said the failure to properly vet Harwood "raised grave concerns".
Scotland Yard has previously admitted the background checks on Harwood were not good enough and is expected to detail later today what changes have since been made.