THEY are the hand-held devices so addictive that they are known as “CrackBerries”. The palm-sized mobile phone with e-mail and internet access allows professional users never to feel out of touch.

Now the taxman is about to dampen their ardour: anyone who sends too many personal e-mails on a work BlackBerry could face a bill of hundreds of pounds, under new rules governing work benefits.

“Anything designed to be used by being connected to or inserted into a computer” will also now be liable for tax, including printers, scanners, modems and even MP3 players, according to Revenue & Customs BlackBerries and other “personal digital assistants” (PDAs) had been exempt from tax because they were used mainly as mobile telephones, but their increasing sophistication has brought them into the taxman’s orbit.

A London-based hedge fund manager said yesterday that any attempt to tax workers for their BlackBerry use would be poorly received. The financier, who admits to using his PDA frequently while sitting down to lunch and dinner, said: “You have got to be kidding. As if they don’t tax the City enough. Is this how Gordon Brown thinks he is going to drum up support from the City?” Setting out the rules in an update to its employee benefits manual, the Revenue said: “Technology has developed to such an extent that PDAs and BlackBerrys [sic] now have additional functions more typically associated with a computer.”

The ruling comes after the Chancellor’s decision in this year’s Budget to scrap the Home Computing Initiative, which allowed employers to give staff up to £2,500 worth of computer equipment tax-free.

Hundreds of thousands of people were able to buy cheap home computers through the scheme but Mr Brown was concerned that companies were using it to give workers bonuses in the form of tax-free widescreen televisions, MP3 players and other electrical luxuries. Under the new rules, the proportion of any work computer equipment used for personal reasons will be subject to a tax charge.

For example, a computer worth £2,500 would attract a total tax charge of £500, or 20 per cent. If the employee made personal use of the computer half of the time, they would be liable for tax of £250. The Revenue insisted that it would not tax anyone whose personal use is “not significant”.

BlackBerry is owned by Research In Motion, a Canadian company. There are more than 3.65 million users worldwide and about 400,000 in Britain, including some of the biggest names in business and politics.

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