From a peak of about 5,400 vehicles that ran on CNG in 2010, the population has dwindled to 4,400 as of the end of last month, reported The Straits Times.

The Land Transport Authority said close to 1,000 owners of CNG vehicles have had their CNG kits removed since 2011. By doing so, the owners pay the Government a pro-rated tax rebate that they enjoyed when they first bought the vehicle.

One taxi operator, Trans Cab, has also fallen out of love with the gas, which is deemed to be more environmentally friendly than diesel. Its managing director, Mr. Teo Kiang Ang, opened the world's biggest CNG refuelling station near Jurong East five years ago.

But less than one-third of his 4,500-strong fleet of CNG cabs are on the roads now and Mr. Teo expects none will be around in another two years or so.

He attributes the dwindling interest in CNG to several factors, including a multitude of restrictions imposed on the transportation and storage of CNG that has made the setting up of new stations less viable. Mr. Teo noted that CNG car owners have also been largely disappointed with their vehicles.

There are now about 2,150 CNG passenger cars in Singapore, compared with 2,500 diesel cars. Back in 2010, the respective numbers were 2,706 and 138.

Other industry watchers said a government duty imposed on the fuel from January 2012, and the replacement of the previous Green Vehicle Rebate with a carbon-based incentive scheme also sealed the fate of CNG.