The National Taxi Drivers' Union has called for "social welfare clearance certificates" to be introduced following the tax-certificate requirement which comes into force tomorrow.
From tomorrow, tax clearance certificates will have to be provided by taxi-drivers before they can renew their public service vehicle (PSV) licence or obtain a new one.
The Department of Transport says it does not expect the requirement will lead to a dramatic drop in the size of the State's taxi fleet, particularly as the requirement is to be phased in over a year as licences come up for renewal.
The National Taxi Drivers' Union is not so sure. Its spokesman, Mr Vinnie Kearns, said he believed a lot of taxi-drivers, not members of his union, were operating in the black economy.
"It is a simple question of maths. There are 16,000 PSV licences issued by the State, there should be 16,000 drivers registered for tax. The Revenue can do the maths; but we don't think the figure is half that."
Mr Kearns said he believed the situation regarding tax was worse outside Dublin where many drivers operate part-time.
Dublin had a serious problem with drivers who were on social welfare as well as drivers who had other, usually daytime, jobs.
"We called for tax clearance certs nine years ago and have repeated that call every year since. But we have also called for social welfare clearance certs and we think they should be required now. It is the next logical step."
Mr Kearns said a driver on social welfare could earn €200 to €300 a week more than other drivers which was an unfair advantage. He also said his union was aware of factory workers who operated an eight-hour day and then drove a taxi at night.
"People have told us they have found drivers asleep in their cars which is seriously dangerous. It also contravenes the working-time directive and that person is a danger to their colleagues during the day and their passengers at night."
Mr Brennan said the introduction of tax-clearance certs was part of his attempt to ensure a quality, reliable taxi service which had the confidence of the public.
He revealed that since deregulation, the number of taxis and hackneys had increased dramatically in all the main cities.
In Dublin there were almost 10,000 taxis at the end of June, compared to fewer than 3,000 before deregulation. In Cork, the number of taxis rose from 216 to 719; in Galway, the number rose from 148 to 469; in Limerick, the numbers rose from 206 to 513 and in Kilkenny, they rose from 17 to 103.
New taxi services have been introduced in Letterkenny, Bray and Cobh among other places.
According to the Minister the increase in taxis has had a corresponding increase in employment in the industry.
