A man has denied the manslaughter of PC Andrew Harper, who was killed after being dragged under a van while on duty.
PC Harper, from Wallingford, Oxfordshire, died after being dragged under the wheels of the vehicle in the village of Sulhamstead, Berkshire, on August 15.
Henry Long, 18, from Mortimer in Reading, and two 17-year-old boys, who cannot be identified, are charged with his murder, an alternative of manslaughter and conspiracy to steal a quad bike.
A fourth defendant, Thomas King, 21, from Basingstoke, is accused of conspiracy to steal a quad bike.
Three of the defendants appeared at the Old Bailey by video link from custody while King was in the dock before Mr Justice Edis.
During the hearing, King pleaded guilty to conspiracy to steal on or before August 15.
He was granted bail until his sentencing at the conclusion of the trial of the other defendants.
One of the 17-year-olds, wearing a grey tracksuit and large crucifix, entered not guilty pleas to manslaughter and conspiracy to steal.
He was not asked to enter pleas to the further charge of murder.
A further plea hearing for Long and the other 17-year-old was fixed for January 7.
A six-week trial has been fixed at the Old Bailey to start on March 9.