Under government pressure to make pump prices transparent, the fuel industry will have to prove in the future that UK drivers are paying a fair price for fuel, the AA said.
In the past month, the average pump price of diesel has fallen 4.99p a litre, from 144.30p in mid May to 139.31 now. But UK diesel costs at least 4p a litre more than in Europe.
More top news
Thirty facts about Prince Harry to celebrate his coming of age.
Django Unchained actress Danièle Watts was handcuffed by police after she was seen "showing affection" to her partner, NBC News reported.
A US news anchor has told viewers he only has four to six months to live on air, after he was informed that his brain tumour was inoperable.