Two teenagers due to go on trial for the murder of Pc Andrew Harper have admitted conspiracy to steal a quad bike.
The pair, who are both 17 and cannot be named for legal reasons, pleaded guilty at the Old Bailey on Monday to plotting to steal the bike.
They and a third man, Henry Long, 18, from Mortimer, Reading, are also accused of the murder of Pc Harper, which they deny.
Long has previously admitted manslaughter and conspiracy to steal a quad bike. The two 17-year-olds deny manslaughter.
They attempted to steal the quad bike from a home near the village of Sulhamstead in Berkshire on August 15 last year.
As they drove away from the property, it is alleged that 28-year-old Thames Valley officer Pc Harper was dragged under a car and suffered multiple injuries.
The case before Mr Justice Edis is due to be opened by prosecutor Brian Altman QC on Tuesday, and go on for up to six weeks.