A police officer was killed in “truly shocking circumstances” when he was dragged for more than a mile along a road by three people trying to steal a quad bike, a court has heard.

Andrew Harper, 28, a police constable with Thames Valley Police, died on 15 August last year of “catastrophic, unsurvivable injuries” after his feet became lassoed in a rope attached to a car, which the defendants had been using to steal the bike.

Three teenagers – two 17-year-olds who cannot be named and 18-year-old Henry Long – deny Harper’s murder. Long, from Mortimer, Reading, has pleaded guilty to manslaughter. They have all admitted conspiracy to steal a quad bike.

Opening the case for the prosecution at the Old Bailey on Tuesday, Brian Altman QC said: “With his ankles caught in a strap that was trailing behind a car being driven at speed along a country lane, he was dragged for over a mile along the road surface, swung from side to side like a pendulum in an effort to dislodge him, losing items of his police uniform along the way, with the rest of his uniform being quite literally ripped and stripped from his body.

“When, at last, he became disentangled, he was left with the most awful injuries, from which he died there on the road, surrounded by colleagues who tried in vain to save him.”

The jury heard that he died naked apart from his socks and boots and some shredded remnants of the trousers he was wearing. “This was a completely senseless killing of a young police officer in the line of duty,” said Altman.

Harper became entangled in the strap as he and a colleague confronted the three defendants on a country lane near the village of Sulhamstead in Berkshire, after responding to a call about the reported theft of the quad bike, even though it was past the end of their shift.

“As [Henry] Long floored the Seat car to make good their escape, PC Harper was lassoed around his ankles by the loop of his strap,” said Altman. “It is the prosecution case that Long drove that car knowing full well that PC Harper was entangled in the strap, as he drove in a manner calculated to dislodge him, and to make good their escape, as had been the plan all along.”

The court heard that Long drove the car along the country lane at an average speed of 42.5mph. Altman said the attempts to dislodge the officer were evidenced by the “snaking trail of apparent tyre marks, abrasion and scuff marks, blood and body matter left behind on the road surface, as well as personal items and clothing that were ripped from PC Harper”.

The car then pulled onto the A4, where another driver had to slam on his brakes to avoid hitting it. “At first [the other driver] thought there was a bloodied deer attached to the car, but quickly realised it was a person, trapped by both ankles, being dragged around the road and striking the kerb,” said Altman.

The prosecution said the idea that the three teenagers did not know Harper had become entangled was ludicrous. “It is not difficult to imagine the screaming and shouting that must have taken place inside that car about what was unfolding,” said Altman.

The trial continues.