The city's transport department has started legal proceedings against Uber, the privately run taxi-sharing service, for operating without a licence.
Uber drivers, who started work in Madrid in September, could be fined €18,000 for operating without the necessary permits and could have their vehicles impounded.
City transport in Spain is regulated by national laws but inspections and sanctions are raised by the regional governments. So far Madrid has started proceedings against 70 Uber operators.
Licensed taxi drivers have already held strikes in both Barcelona and Madrid against Uber but some of them are finding, as in other European cities, that they can make more money if they also join Uber, especially in their off-peak hours.
Uber uses private drivers and private cars to provide a taxi service via its special app. The main complaints against Uber is that the fares undercut those of official taxis and that the cars used in Uber operations are potentially unsafe because they are not subject to the usual safety inspections of licensed taxis.