The Office of Fair Trading (OFT) is to report on a review it has been carrying out into the petrol market.
Campaigners have called for the watchdog to announce a full investigation into the sector, saying there needs to be greater scrutiny.
The review, launched in September, has been looking at whether reductions in the price of crude oil are being passed on to motorists.
The OFT called then for information from the industry, motoring groups and consumer bodies amid concerns over the prices charged for petrol and diesel at the pumps.
The founder of campaign group FairFuelUK, Peter Carroll, said on Tuesday that the OFT should call for "a landmark full investigation into why the UK road fuel market is so dysfunctional with its regional variations, slow price falls, apparent market monopolies and pump prices that are measured in nine-tenths of a penny".
He added: "There is a suspicion that some people are making a lot of money speculating in the petrol and diesel market that is essential for every family and business in the country. This needs to be fully examined and these suspicions addressed.".
Quentin Willson, national spokesman for FairFuelUK, said: "This market has been hiding itself in the long grass for far too long and consumers have a right to transparency, honesty and fair pricing.
"We want to know how wholesale prices are calculated, why wholesale prices move, why it takes so long for falls in crude oil to appear at the pumps, why diesel is more expensive than unleaded and why some fuel retailers can charge up to eight pence more per litre than others often in the same area. It's mischievous that this market has stayed so resistant to scrutiny for so long."
The OFT says the UK retail road fuels sector is estimated to be worth around £32 billion. Petrol prices rose by 38% between June 2007 and June 2012, while diesel prices went up by 43%.
The watchdog said on launching the review that it would explore a number of claims about how the sector is functioning, including whether supermarkets and major oil companies are making it more difficult for independent retailers to compete. The review would also consider whether there is a lack of competition between fuel retailers in some remote communities.