An investigation by the BBC’s Panorama programme claims to have uncovered evidence that companies such as GSK and Northern & Shell exploited tax loopholes to dramatically reduce the amount they paid to the UK exchequer.

The programme was passed confidential tax agreements, devised by the accountancy firm PriceWaterhouseCoopers, that show how the firms set up offshore subsidiaries in Luxembourg – which then loaned money back to their UK operations.

This let them offset the loans against corporation tax in Britain while paying a tax rate on the profits in Luxembourg of as little as 0.5 per cent. There is no suggestion that any of the schemes were illegal.

Northern & Shell, owners of Channel 5, the Express newspapers and OK magazine, transferred loans totalling £804m to Luxembourg. The end result was that the company had sheltered profits which would otherwise have generated £6m in UK corporation tax.

Richard Brooks, a former investigator with HMRC, said: “The company puts its money into Luxembourg and borrows it back. It just sends money round in a circle and picks up a tax break on the way.”

In a statement Northern & Shell said: “Our strategy is to comply with relevant regulations while minimising the tax burden for Northern & Shell and our customers. The board considers it entirely proper that Northern & Shell endeavours to structure its tax affairs in a tax-efficient manner.”

Richard Murphy, a tax expert, said the arrangement was legal but he was not sure it was “acceptable”.

“This is purely artificial structuring which is designed to undermine the tax revenues of the UK,” he said.

PriceWaterhouseCoopers, which has given more than £1m worth of advice to both Coalition parties and Labour, said its help “is given in accordance with UK, European and international tax laws and agreements”.