New rates published last month, just days before they would be introduced, increased the fuel scale charge - the tax on drivers receiving free fuel for private mileage - by 16%.
Some drivers will have to cover more than 25,000 private miles a year to make the 'perk' worthwhile. Most drivers who have yet to opt out of the benefit have now run out of time and will incur the full scale charge for the year.
Martin Hender, director of fuel at ARVAL PHH, said: 'Companies need to 'crunch the numbers' on free fuel for each of their drivers. Not only are every driver's circumstances different, but companies must understand the impact of the National Insurance contributions they pay on top of the cost of the free fuel.
'Even more importantly, if companies do decide to take away free fuel they must hang onto their fuel cards. Fuel cards are the most effective means of keeping business and private mileage separate, as well as providing the in-depth information vital for managing fuel costs effectively.'
Leading figures in fleet have been rushing to help fleet drivers make a decision. Roddy Graham, managing director of contract hire and vehicle management firm, Ignition Vehicle Management, this week launched the company's fuel tax calculator service which enables drivers to ascertain whether to accept free fuel or opt for a cash alternative.
He said: 'Over the past few months, much of the focus within the market has been on the implications of benefit in kind taxation based on Co2 emissions which has, perhaps understandably, clouded drivers' awareness of the changes in free fuel taxation.'