Gas and electricty giant Npower has avoided up to £108m in UK corporation tax over the last four years by squirelling money back to its parent company in Germany via Malta.

It does this by ensuring that over half of the funding that emanates from German owner RWE is derived from loans which are then paid through a shell company based in the Mediterranean tax haven, according to The Sun. The newspaper, which based its findings on research from Richard Murphy, head of Tax Research and a founder member of the Tax Justice Network, said Npower then pays back the annual interest on the loans to the Maltese company, Scaris, allowing it to post a loss there and avoid corporation tax.

Murphy said that by employing such accounting measures 'Npower and its German group have saved more than £60m a year in tax over four years by using this structure and maybe £100m of tax in the UK has not been paid as a result of using it.'

'It is very hard to explain why RWE would lend money to Npower via Malta unless tax avoidance was its motive,' he continued. 'The tax savings that results are very clear.'

RWE has paid around £2.3bn in loans to npower via Scaris since 2008, according to company accounts.

'It's like a husband borrowing from his wife and paying it through the children to get a tax break on the way,' said Murphy. 'Let's not pretend for a minute it's the same as a commercial deal.'

And he said the case was 'remarkably similar to Starbucks'.

Two week s ago, its chief executive, Paul Massara, claimed the company did not have to pay corporation tax in 2009, 2010 or 2011, because it has got relief for investing in new wind farms and power plants.

An Npower spokesman said: 'This report doesn't add anything new. We've done nothing wrong and HMRC are aware of everything here and they still class us as a low risk company. Using Malta made no difference to our UK tax contributions and we've not used this route since last year.

'It's sad to see this continued harrying of a company like ours that has invested billions into the UK. The number of informed commentators who have defended our tax position in the last two weeks, shows that this criticism is flawed.'

Pressure group 38 Degrees - who commissioned the report - started a petition on its website last week calling for the utility giant to stop dodging its 'fair share of tax in the UK'. So far it has amassed 106,000 signatures.

In November, Npower - which has 7m UK customers - raised its average prices by £112 per household while its profits rose by a quarter to £390m.

An HMRC spokesman said: 'For legal reasons, we can't comment on the affairs of individual taxpayers.'