About 34,000 private-hire car drivers - who have been driving for the Uber and Grab apps - have yet to undergo the required vocational training or pass the course's test.

They will have three more months - until the end of June - to do so, failing which they will have to stop driving for fares.

This figure, released by the Land Transport Authority (LTA) yesterday, has elicited concerns from the National Private Hire Vehicles Association (NPHVA) about the adequacy of training spaces.

With the phasing in of a new vocational licensing regime for the industry, these 34,000 drivers were among those given a one-year grace period by the LTA to continue providing chauffeur services.

The concession was granted to those who had submitted a licence application before the cut-off date of June 30 last year.

The LTA said 17,000 drivers have completed the 10-hour Private Hire Car Driver's Vocational Licence (PDVL) course and passed the test so far.

In the last five months, an estimated two out of three drivers who took the PDVL test passed, an LTA spokesman said. The pass rate is similar to that for the vocational licensing course for taxi drivers.

The LTA said that to cater to the new demand from applicants, it appointed ComfortDelGro Taxi as an additional training provider, and the Employment and Employability Institute as an additional test centre in December.

The Singapore Taxi Academy started conducting the PDVL course and test in April last year.

Still, NPHVA executive adviser Ang Hin Kee said: "It will be worrying if there is inadequate capacity and many have to wait their turn to take the course and test."

Mr Ang also said some drivers have given feedback that they had to queue to get a class slot or test date.

When asked if it was concerned about a potential dip in driver numbers after June 30, Grab did not comment. But the company said it encouraged all Uber and Grab drivers to give themselves sufficient time to study and prepare for the test.

National University of Singapore transport researcher Lee Der-Horng, however, surmised that private-hire car drivers now on the roads and have yet to take or pass their PDVL test may be part-timers who are weighing their options.

Given the recent Grab-Uber merger in Singapore - which may lead to driver incentives being reduced - Dr Lee said some may leave the industry entirely.

He believes that until Indonesian ride-hailing giant Go-Jek makes its debut here, the number of private-hire drivers may dip, as compared with the days when both Uber and Grab were active and recruiting drivers.