All police forces in the country will be made to examine their archives to search for evidence of misconduct by undercover officers following allegations surrounding the Stephen Lawrence murder inquiry.
Policing minister Damian Green admitted the probe could lead to "some very unpalatable truths" coming out, but said it was better than keeping them hidden.
He is expected to tell the chief constables of the 43 forces in England and Wales they must carry out the searches following the revelation that an undercover officer was told to find information to use to smear the Lawrence family after the teenager's racially aggravated murder in 1993.
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Peter Barnett made hundreds of trips from his home in Oxfordshire to Marylebone station without paying the full fare.
Police say he may have attacked more children and urged them to come forward.
The suspect walked up to a Tesco employee and shoulder barged him to the ground before punching him repeatedly in the face.