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The findings were discovered by a team of researchers across the globe, from universities in the UK, US and Australia, and co-ordinated by BT. The team were given hard disks purchased second hand by BT, without knowing where they were from.

They then carried out a forensic imaging process to have a look at the drives. If companies disposed of data correctly, customers details should have been overwritten with random data, so that they're clean. The researchers found that this was not the case with 66 per cent of hard drives they encountered.

"Corporations are failing to dispose of data correctly. They then leave themselves open to suggestions that they've not fulfilled their obligations on the Data Protection Act and have got customers data and details," said Dr Iain Sutherland, reader in computer forensics at the University of Glamorgan's Information

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Security Research Group.

"We've found a whole range of material from individuals' medical records, through to private companies to data that may have originated at a large multinational company. We had one that contained financial records in millions for one company."

He admitted that in the wrong hands, criminals could access customers' bank details from second hand computers and steal funds, due to a company's failure to wipe data correctly.

"In theory, if the bank hadn't disposed of it correctly, yes - it's quite possible," he said.

Sutherland also offered advice to consumers who are looking to sell their own PCs to the second hand market. He warned that simply deleting data by moving it to the recycle bin and emptying it, doesn't work.

He advised that users need to remove that information to protect their data, and there are a variety of tools you can use, including some free ones that the researchers used to overwrite hard disks.

"The example that we tend to use when teaching some of the courses here, is that we say that you should treat your computer hard disk like a video tape," Sutherland explained.

"You may write across the label: 'I can reuse this' or that it's blank, but unless you've taped over what's actually on there, the content is still there."

Earlier this year, the team found that sensitive data had failed to be wiped off one on five second-hand mobile phones.