Taxis aged up to 19 years old are working in Northampton following a relaxation of rules on the age of vehicles.
The borough council changed the three-year age limit for hackney carriage last month, letting drivers make a living from any age of cab.
Northampton Hackney Carriage Association said it was concerned that already dwindling numbers of spaces on the ranks would become fewer still.
In the last three years, numbers in Northampton have already jumped from about 125 to 150.
Joe Matthews, the chairman, said although a handful of drivers had already started up with cabs nearly 20 years old, the issue was not poor quality vehicles.
He said: “All cabs have to pass strict tests. The issue is too many hackney cabs competing for too few spots.
“The rule change means they are far cheaper to buy and we’ve already seen a few extra ones arrive since last month. It means you have to wait a good hour after a job to get back on a rank and we all end up working longer hours to make up earnings.”
Mr Matthews said the rules meant there was not a level playing field between established drivers who have recently paid £35,000 for a new vehicle and someone just arriving in the trade who has paid £2,000.
The borough council stressed that all cabs that were licenced had passed strict test.
A spokeswoman said: “Prior to January 2016 there was an upper limit on the age of hackney vehicles at first registration. This was finally removed following a committee decision in 2012 to phase it out.
“The situation remains that all vehicles being registered as Hackneys and private hire vehicles have to pass a very stringent inspection prior to be being licensed and the council must be satisfied that the vehicles used in Northampton are safe.
“We have not seen a significant increase in the number of vehicles being registered since this change came into effect.”