It is against the rules for drivers of private-hire cars, such as those under car-booking apps Uber and Grab, to pick up passengers off the streets without prior booking. But some still try their luck anyway.

The National Taxi Association said some private-hire drivers are picking up fares from the kerbside as well as at taxi stands. While passengers do not seem to mind, the association's Executive Adviser, Mr. Ang Hin Kee, said the taxi group has raised this issue with the authorities, and it has been told enforcement actions were being taken.

In response to queries from The Straits Times, the Land Transport Authority (LTA) said last week it has investigated 35 alleged cases of private-hire vehicles and private cars picking up fares illegally since 2013.

"Out of the 35 cases involving unlicensed private cars or private-hire cars that the LTA has investigated, one has been charged and 19 were issued with warnings," a spokesman said. She said the rest were found to 'not have infringed any relevant rules' after investigations.

Taxi driver Alan Tang, 53, said such cars have been seen at several places, including nightspots and shopping centres. "If I were a private-hire driver desperate to cover my rental, I would wait by the side of the road," he said. "People will approach me when they see my Uber or Grab logo. I am not touting or soliciting, so I don't think I am breaking the law. That is a loophole there."

Two weeks ago, Facebook user Mohamed Hanas posted images of a new Toyota Sienta outside Mustafa Centre at 1:00am. "Blinking 'TAXI' sign on driver side," he wrote. "Uber logo on passenger side. Parked the car with passenger door open... looking at passers-by." It was the second such posting in recent months. There was another post on a private car spotted in a taxi queue at Changi Airport. Recent checks by The Straits Times found a number of private cars waiting along slip roads leading to the airport terminals.

The LTA did not say if it is illegal for a non-taxi to carry a Taxi sign, but said all private-hire vehicles are not allowed to pick up street hails like a taxi. "Anyone who operates an unlicensed public service vehicle, or causes or permits a motor vehicle to be used as one, is liable to a fine not exceeding $3,000, or imprisonment for a term not exceeding six months, or both," it said. "The vehicle used may also be forfeited."

Lawyer Bryan Tan, 45, said if private-hire vehicles were allowed to pick up fares from the kerbside, "I'd have no qualms getting into one". "My concern, however, will be kids and unaccompanied women," he said. "For them, they need to be a bit more conservative."

In April, Senior Minister of State for Transport Ng Chee Meng said private-hire drivers will be subject to regulations, to safeguard the interest of commuters.

Like taxi drivers, they will have to go through medical tests and background screenings, attend a vocational-licence course, and be put under a demerit point system.

Private-hire cars used for bookings under Uber and Grab - which made their debut in Singapore in 2013 - will have to be registered with the LTA, and the vehicles must display decals prominently so that commuters and the authorities can easily identify them. These rules are expected to kick in in the first half of next year.