The Independent Police Complaints Commission will oversee an investigation into the alleged leaking of information surrounding the Andrew Mitchell "plebgate" row.
The IPCC said it had decided to supervise part of the Met's investigation - surrounding the "validity" of a serving officer's claim to have seen the incident on the 19 September.
A statement said:
The investigation is considering the validity of the officer's claim, which is understood to have been made to a Member of Parliament in a private capacity.
The investigation is linked to enquiries by the MPS into how internal MPS information was obtained by national newspapers following the incident.
The IPCC received a referral from the MPS today and following an assessment of the available information, a decision was made to supervise the matter.
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Andrew Mitchell has written a piece in which he has accused the police of trying to destroy his career in a smear campaign.
Police investigating the row involving Andrew Mitchell have arrested a second man today. See how the "plebgate" events unfolded.