Some 42,900 private-hire car drivers applied for a new mandatory vocational licence before July last year and got a grace period of one year to get the licence.
But only half attained it before the concession period ended last Saturday.
This means that about 21,000 of these drivers are no longer able to drive for private-hire car services such as Grab.
In response to media queries, the Land Transport Authority (LTA) said on Thursday that only 22,000 - 51 per cent - of those who applied for a Private Hire Car Driver's Vocational Licence (PDVL) before July 1 last year passed the test.
Another 3,100 drivers took the required course but failed the test, while 900 took the course but did not sit the test.
Some 16,900 - or about 40 per cent of all applicants - did neither the course nor the test.
Grab declined to say if it has seen a significant drop in the number of active private-hire car drivers in recent weeks.
But the ride-hailing firm said it was working with the Employment and Employability Institute to find job opportunities for drivers who did not get the licence. It added that drivers could choose to do deliveries through its food delivery service GrabFood, which does not require a vocational licence.
A Grab spokesman said the firm was also working with the LTA to support drivers who still wish to take the PDVL, as well as exploring other options for the drivers.
The taxi sector has seen a slight resurgence - the number of taxi driver vocational licences issued increased from 77 last December to 410 in April this year.
Also, figures from the LTA showed that only 9 per cent of Singapore's taxi population were not hired in April, down from 12.5 per cent in January.
Mr Ang Hin Kee, executive adviser to both the taxi and private-hire vehicle associations, said the number of PDVL holders did not necessarily reflect accurately the number of private-hire car drivers on the road.
Noting that the vocational licence for taxi drivers could also be used to drive private-hire cars, he said: "The cost of taxi rental remains quite high, so some taxi drivers, meanwhile, choose to drive private-hire vehicles."
But Mr Ang, an MP for Ang Mo Kio GRC, said that he expected fewer people to take up the job of driving private-hire cars in the future with Grab rolling back its incentives for drivers.