Energy experts expect tax collection for diesel will rise by another three baht a litre in the fourth quarter amid a continued decline in the global oil price.
Manoon Siriwan, an energy analyst, said the government should raise the combined excise and municipality taxes on diesel by three baht a litre.
Given the recent drop in the global oil price, now is the right time to resume tax collection on diesel without affecting the fuel's retail price, he said.
Mr Manoon estimates the government is losing almost 100 billion baht a year in tax revenue.
The global oil price has declined by US$10 a barrel over the past two weeks.
As part of the government's energy reform policy, the Energy Ministry recently decided to recollect the combined tax at 82.5 satang.
Mr Manoon, who is also a member of the National Reform Council, said since 2008 the two taxes totalling 5.31 baht a litre had been waived, leaving only half a satang, in order to keep diesel below 30 baht a litre.
Adjusting the price of liquefied petroleum gas (LPG) will be the next step of energy reform in order to reduce the financial burden of subsidising the state Oil Fund, he said.
Mr Manoon estimates energy subsidies between 2008 and last month hit 920 billion baht.
Of that amount, the diesel subsidy accounted for 370 billion baht.
Deputy energy permanent secretary Kurujit Nakornthap said the goal of adjusting the diesel price was to narrow the price gap between petrol and diesel in order to balance fuel consumption.
Over the past two years, diesel consumption has risen to more than 60 million litres a day, a record high, due to price distortion.
Before the diesel price was capped, daily consumption was 55 million litres.
Capping the price of fuel not only distorts consumption but also lowers consumption of biofuel since fossil fuels are cheaper.
Mr Kurujit said the new National Energy Policy Council would consider how to lift the universal subsidy programme for LPG and make a direct subsidy to low-income earners.
The price is likely to be capped at 18.13 baht a kilogramme for this group.
Last year, the LPG subsidy for low-income earners stood at only 8 million baht compared with 7 million for the first nine months of this year.