Leicester cabbies have accused council bosses of not doing enough to enforce rules on how fares are picked up.
The Leicester branch of the Rail Maritime Transport (RMT) union, says officials are ignoring the issue of "flimping" or "touting" where private hire vehicles illegally pick up passengers.
Under current rules only black hackney cabs are allowed to collect passengers on the street without pre-booking.
However the RMT, which represents 210 of the 325 registered hackney cab drivers in the city, says the rules are being regularly flouted.
The union says Leicester City Council, which licenses cabs, is turning a blind eye to the issue.
A union spokesman said: "They know it is happening because our members are reporting it to them and giving them evidence.
"They don't do anything though.
"It is a serious safety issue because sometimes there are cars without plates picking people up.
"They won't have had any safety checks done on them. They may not even be roadworthy or insured to carry passengers."
Flimpers often undercut the fares of hackney cab drivers.
The RMT is also complaining that the council has not provided enough ranks for taxis.
It has threatened to stage protests if the council does not crack down on flimping and bring in more ranks in busy areas like Braunstone Gate.
It has declined to say what those protest may involve.
RMT acting general secretary Mick Cash said: "The licensed black taxi trade in Leicester is furious at what RMT perceives to a campaign to undermine and harass our members, a campaign which can be tracked all the way to the front door of the city council.
"The level of anger amongst the black cab drivers is unprecedented and RMT is launching a campaign to end the abuse of the plying for hire regulations, the proliferation of bogus mini-cab touts and the shocking lack of ranking provisions for the legal and licensed trade which encourages the dangerous abuses of the system.
"RMT is looking at staging protests in Leicester if the council continue to fail to take our members concerns seriously."
Assistant city mayor Piara Singh Gill said: "I meet the taxi drivers on a regular basis so I am very surprised to hear that the RMT members feel the city council isn't taking their concerns seriously.
"The city council carries out regular exercises to crack down on flimping – the illegal pick-ups made by private hire mini-cabs – and where there is sufficient evidence, we do prosecute. Our experience, however, suggests that the practice is not as widespread as the black cab drivers perceive it to be.
"Following consultation with the licensed taxi trade, we have retained the restriction on the number of licensed hackney carriages on the road, relaxed our policy on vehicle age from eight years to 11 years, increased controlled fares, and set up a council-run vehicle testing station.
"We have also told the drivers of another benefit we intend to launch, which would see taxi marshals on the street as part of the business improvement district initiative.
"These ideas have emerged during our discussions with the drivers."
He added: "Over the last 18 months, we have created more than 60 additional spaces at ranks across the city centre and commissioned a review of the city's taxi ranks to help identify areas where new ranks could be provided.
"We have also considered all of the suggestions for new ranks that have been put to us by the taxi drivers, but clearly any new rank has to be in a suitable location and must not be an obstruction to other road users, emergency vehicles or pedestrians."