Friends initially tried to rescue him and after they lost him in the deep waters they raised the alarm.

Fire crews from both sides of the county boundary raced along narrow, country roads to get to the scene. They included the water rescue team from Nelson Fire Station, pumps from Nelson and Colne and a major incident support unit with a boat from Preston.

The young people – believed to have been staying in a hostel in the Trawden area – had gone to the reservoir on the moorland route from Pendle to Hebden Bridge for a picnic and swim.

The 17-year-old had been swimming for a while at the Lancashire end of the reservoir when he got into difficulties. His friends tried to rescue him but he disappeared under the water. It is understood they ran up the hillside to the road and flagged down a passing car to call for help.

Station Manager Dave Jackson, of Nelson Fire Station, said: "West Yorkshire also sent a water rescue team and we made a systematic search around the point of entry and his last known whereabouts using specialist equipment from both fire and rescue authorities. The initial report was he had been in the water for some time."

After searching the area, the teams had been unsuccessful in finding the victim, who is understood to be from London.

The fire and rescue teams then handed over to West Yorkshire Police's underwater search unit. After four hours of searching, they recovered the body of the teenager.

A spokesman for Yorkshire police said the initial call was at 1-35 p.m. and it was 6-20 p.m. when the victim was found. Both the fire and rescue service and police have warned people to avoid swimming in open waters.

Station Manger Ben Norman, of Nelson Fire Station, said: "We would ask people to bear in mind that there is free swimming pool access for young people in Pendle. And more people die in inland waters than they do from fires."