Xenon is a 'super-snooper' computer system which trawls the Internet looking for patterns, so if a website is selling goods, it'll be picked up. Once a site has been identified, the taxman can cross-check whether revenue has been paid on its trading activities.

The system is currently being used by the Dutch tax authorities and has been exported elsewhere.

Mike Warburton, of accountants Grant Thornton, told This Is Money:

"This sounds like an electronc version of what the Revenue has long since done, flicking through local newspapers, checking business advertisements against tax records."