Up against stubborn competition from private-hire car services Uber and Grab, more cabbies have been giving up their taxis this year.

For the first five months of this year, the average proportion of the taxi fleet which is sitting idle in yards - also called the unhired rate - hit 9.1 per cent.

This is nearly double the average unhired rate of 5 per cent in the same period last year, according to data the Land Transport Authority (LTA) shared with The Straits Times recently.

While the population fluctuates as vehicles have to be renewed every eight years, this translates to around 2,400 taxis being unhired, up from about 1,300 previously.

As of April, there are 26,476 taxis run by five operators: ComfortDelGro, Trans-Cab, SMRT, Premier and Prime.

ComfortDelGro, which operates the largest fleet - more than 15,860 cabs under the Comfort and CityCab brands, said its unhired rate has remained "relatively low" despite stiff competition, though it declined to provide a number.

The firm rolled out a flat fare booking option in April and also recently updated its rewards programme to target different customer groups.

Since late last year, firms such as Trans-Cab and Premier have been cutting rentals by up to 40 per cent to attract hirers.

Mr Neo Nam Heng, chairman of the Prime Group, which owns more than 700 cabs, said that about 10 per cent of his fleet is not rented out.

National University of Singapore transport researcher Lee Der Horng expects the unhired rate to increase further as more commuters accept private-hire car services, and cabbies themselves switch to becoming private-hire car drivers.

For cab firms, the costs of having one idle taxi requires about seven hired taxis to cover, he said. "If the unhired rate surges to double digits, it may not be sustainable for some firms," Dr Lee added.

LTA figures show that between January and April, the average daily ridership for taxis was at an eight-year low of 853,000, a 12 per cent drop from 977,000 in the same period last year.

Cabby Thomas Yuen, 40, said business is particularly poor on weekdays after midnight, as passengers tend to take private-hire cars, which do not levy a surcharge.

"It can be very challenging. I've roamed around for an hour without any passengers," Mr Yuen said.

Prime's Mr Neo feels that private-hire car services will have to turn in a profit some day, and will eventually cut back on the generous discounts for passengers and incentives given to drivers. He said: "I am confident that while the taxi business is down, it is not out."