Staff at a computer store in Oxford have been accused of failing to diagnose basic computer problems and overcharging for repairs.

The PC World branch in Botley Road misdiagnosed a computer with a loose cable as having a corrupted hard disk - and quoted £300 for a repair, according to the consumer organisation Which?

Staff from its magazine Computing Which? anonymously visited 20 computer stores - including seven PC World stores and 13 independent outlets.

The magazine said: "PC World is advising customers to fork out hundreds of pounds for unnecessary repairs, or to scrap perfectly good PCs. It needs to address this situation urgently."

Abigail Waraker, the magazine's editor, added: "It's shocking that simple problems, such as a loose cable, can be misdiagnosed and stores are getting away with charging for their mistakes.

"PC World is the only big chain that repairs PCs it didn't sell, making it the first port of call for many owners. They need to overhaul their training."

Two branches of PC World and three of the 13 independent stores visited by researchers opted to reinstall Windows in order to fix a software fault. This would mean customers losing data which had not been backed up.

On six occasions, PC World branches refused to even look at PCs because they were not returned with a boxed copy of Windows or a recovery disc. But Computing Which? said the majority of the 20 stores it visited provided good repairs.

Prices were generally lower at the independent stores - with the majority charging less than £100 to fix the faults.

Researchers visited each of the outlets twice - once with the software fault and then with the loose lead.

They deliberately "broke" computers beforehand by deleting a file that helps to start up the Windows system and loosening a cable connecting the hard disk to the rest of the computer.

In one London branch, staff misdiagnosed the software problem as a corrupted hard disk and quoted £350 for repairs. A member of staff even advised buying a new PC - at a cost of £500.

Hamish Thompson, a spokesman for PC World, said: "PC World's skilled engineers have fixed more than two million PC problems for customers in the last year and in nine out of 10 PC repairs, there was no requirement to fit a spare part.

"We're constantly improving the quality of our services and we're grateful to Which? for their feedback on 14 visits."