The incident is the latest in a spate of thefts from sports grounds and police are warning users to be on their guard after 10 separate incidents in the Oxford area since January.

Paul Lyon, 41, from Cumnor, was refereeing an Oxfordshire County Cup semi-final at the Horspath Road Athletic and Sports Ground on Sunday afternoon when the thieves struck.

After searching the unlocked officials' changing room they made off with the keys to Mr Lyon's company van, where he had left his wallet and phone.

Mr Lyon said: "They ransacked the vehicle then dumped it a mile and a half up the road."

His wallet was found nearby but cash and credit cards had been stolen.

Mr Lyon, who has been a referee for more than 10 years, said it was not the first time thieves had struck during a match.

"I have had people rifle through my bags," he said.

"Usually I don't take much stuff with me, but on Sunday I had a game in the morning and in the afternoon."

Mr Lyon, who is also the fixture and referees secretary for the Oxford Mail boys' league, said theft was a growing problem.

"In the course of a season four or five referees have kit stolen. For the teams it's more like a weekly occurrence -- everything from mobile phones to change in coat pockets."

The sports ground is maintained by Oxford City Council and facilities manager, Ian Thompson, said policing sports grounds was hard.

"We have so many repeat users it is difficult," he said.

Mr Thompson also said that users were encouraged not to leave belongings in changing rooms and that coin-operated lockers were in operation at the Horspath Road ground.

Thames Valley Police are investigating the incident and have urged users not to leave valuables in changing rooms.

Oxford United players had mobile phones and jewellery stolen from changing rooms at the Kassam Stadium last August.