Police are warning learner drivers to ensure instructors are properly qualified, after roadside checks found some giving lessons were unlicensed.
The Metropolitan Police told the BBC that if people were not taught properly it could impact long-term road safety.
The Driving Standards Agency has undertaken 30 roadside operations since 2011 to target illegal instructors in England, Scotland and Wales.
On average a suspected illegal instructor is reported to it daily.
Vasim Choudhary, a DSA fraud investigator, said: "They sticker up their car to look like reputable driving schools, and candidates presume they are legal when they are not.
"A qualified instructor will teach you skills which will set you off on your driving career, so you can be a safe driver for life, as opposed to someone who might just teach you skills which are very limited," he said.
The police are also worried that illegal driving instructors may not have been through the obligatory criminal background checks.
Insp Vince Brady, of the Met roads unit, said: "We talk an awful lot with people around using a licensed minicab, because you've got confidence that that vehicle, that person, is subject to a process and a series of checks.
"It's exactly the same with learner drivers. They're getting into a vehicle with a stranger they may know nothing about."
In the past four years, 123 suspected illegal instructors have been arrested, of whom 39 were convicted.
However, the DSA said it is often hard to secure convictions because it has to be proved that money has changed hands between learner and instructor.
In Northern Ireland the Driver and Vehicle Agency and the Road Traffic Policing Division of the PSNI have investigated five cases in the last three years.
More than 41,000 approved and trainee driving instructors were working in England, Scotland and Wales in 2012-13, and all DSA-registered instructors giving lessons to learners have to display an in-date badge on their windscreen.
A green badge shows that the instructor is fully qualified and undergoes regular DSA checks.
A pink badge indicates a trainee who can teach for six months while acquiring teaching experience.
The badges should carry the instructor's photograph, a unique reference number and an expiry date.
Learner drivers are now being urged to check their instructor's qualifications before getting into the car.
Removed from register
BBC Radio 5 Live Investigates recently followed police and DSA officers on an operation to catch rogue instructors in a part of east London.
During a series of roadside checks, vehicles displaying L-plates or driving school logos were stopped and instructor accreditations checked.
One instructor was pulled over after officers noticed he was not displaying a badge on his windscreen.
The man claimed to be going through an appeal tribunal but officers later discovered he had been removed from the DSA register in 2011 because he had penalty points on his own driving licence.
The learner driver in the car said she had not thought to check his credentials.
"He taught my mum, and because they knew each other quite well he was passed on to me, so I didn't really bother," she said.
"I'm quite shocked because I didn't think this would ever happen."
The man was arrested on suspicion of fraud and driving without insurance and had his car impounded.
Similar police operations have taken place in Blackpool, Stirling, Derby, Bradford, Dundee, Chingford, Bishops Stortford, Coventry, Newcastle-under-Lyne, Leicester, Birmingham, Aberdeen, Glasgow, Nottingham, Stafford, Rochdale, and Manchester.
Insp Brady said: "They are defrauding the people who believe they are a licensed instructor. As well... [there are] safety issues associated with people not learning properly and that impacts on the long-term safety on our roads."