Shadow Policing Minister David Hanson has said a "full investigation" into Plebgate is needed.
He said: "It is vital that a full investigation gets to the bottom of this extremely serious issue surrounding what happened at the gates of Downing Street and in the days after as soon as possible.
"The Prime Minister and the Cabinet secretary should have established a credible investigation at the start, as we urged them to.
"It remains a concern that by treating this simply as a media handling issue No 10 and the cabinet office have made this much worse."
Britain's most senior police officer has promised a "ruthless" investigation into an alleged conspiracy against Andrew Mitchell.
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.