This year has been “the most volatile” for fuel prices since reliable records began, the competition watchdog has found.
The Competition and Markets Authority (CMA) said prices rose by around 50p a litre from January to July – the largest leap recorded within a year – before falling by 31p for petrol and 14p for diesel.
The CMA said this could be due to “other cost rises for retailers or weaker competition on fuel”.
The watchdog also found that the gap between diesel and petrol prices has become “larger than ever reliably recorded”.
Diesel now costs around 24p more per litre than petrol.
This is largely due to western Europe’s reliance on imports of diesel from Russia, according to the CMA.
The watchdog’s interim chief executive Sarah Cardell said: “It has been a terrible year for drivers, with filling up a vehicle now a moment of dread for many.
“The disruption of imports from Russia means that diesel drivers, in particular, are paying a substantial premium because of the invasion of Ukraine.
“A weaker pound is contributing to higher prices across the board too.
“There are no easy answers to this.”
Ms Cardell added that the CMA is considering whether a “lack of effective competition” within the UK is “making things worse” for drivers.