Since deregulation, almost anyone can purchase a taxi plate for an average of 6,300. Between 35 and 40% of the value of a car used as a taxi can be written off over three years. The higher the value of the car, the more money can be written off in tax. A three-litre engine car, like a Bentley, costing around 80,000 would have a massive regain on a tax bill. "On an 80,000 car you could write off over 48,000 over two years, " said Tommy Gorman, president of the National Taxi Drivers' Union, who is aware of the practice. "There are three or four aspects where you would have a big saving, " Gorman said. Other savings can be made on car tax, which stands at 72 a year for taxi drivers, instead of around 1,500 for a luxury car of a normal driver.

Cars with taxi plates can also benefit from beating city traffic by using bus lanes, a practice highlighted when it was discovered that Ryanair chief executive Michael O'Leary had bought a taxi plate for this purpose. "That's a big advantage, but they're breaking the law if they're not actually working as a taxi driver, " Gorman said. There is at least one Bentley using a taxi plate in Dublin, and Gorman says he knows of other cases where wealthy professionals purchase the taxi plates. Plates have dropped massively in value from around 80,000 to just over 6,000 since deregulation. "I've heard of a solicitor in the north Dublin area who puts a roof sign on his car every morning to drive all the way into the city using the bus lanes, " he told the Sunday Tribune.

It is unclear whether people who are buying taxi plates for this purpose are actually using their cars at all as taxis.

"It's definitely not playing from the same deck, " Gorman said. "They wouldn't be welcome in any organisation of ours."

Taxi regulator Ger Deering was more cautious about any abuse that may be occurring. "The Revenue will be looking for the evidence of the business, " he told the Sunday Tribune. "You couldn't write off your tax from an income you didn't have. You would have to show an income. A tax write-off is only good to you if it is against the cost of something.

The tax man is not foolish."

The regulator told the Sunday Tribune that measures to improve customer service and combat fraudulent drivers will be introduced on 25 September. "Coming in to force this month and in the future are a tamper-proof disc which will identify the car as a taxi at all times, as well as permanent in-vehicle information."

Deering will also appoint enforcement officers to ensure that if a driver is not picking up a member of the public, he is refusing to do so because he already has a callout or previous booking, and is not merely driving around under the guise of being a taxi driver. All of these measures will "make it difficult for people who are not genuinely running a business, " Deering said.

A spokesman for the Revenue Commissioner said taxi drivers were eligible to write off 40% of their cars' value as a "wear and tear allowance."

The spokesman said a cash-based business like taxi driving "presents a challenge to Revenue and would be relatively high risk in terms of shadow economy practices although it's important to stress that the vast majority wish to pay their fair share.

"Revenue's strategy in relation to all sectors of the economy, including taxi drivers, is a simple one . . . to promote voluntary compliance, " the spokesman said.