The cost of filling up at the pumps has edged up over the last month, with diesel drivers getting a worse deal than those using petrol, according to figures from the AA.
The average price of petrol in the UK has risen from 133.35p a litre in mid-May to 134.61p in mid-June, while diesel has gone up from 138.17p a litre to 139.16p.
Northern Ireland has the most expensive petrol, at an average of 135.8p a litre, with London having the cheapest, at 134.61p.
Northern Ireland also has the dearest diesel (139.8p a litre) with London and south-west England having the least expensive (139.1p).
The AA said the slight rise in average petrol prices nationally represented "something of a lull" after the 8-10p swings in prices over the last 12 months. But it warned that this year retailers have on average been "creaming up to £1 a tank extra off diesel car drivers and up to £1.40 a tank extra off diesel van owners".
The AA continued: "At present, the 1p-a-litre premium that fuel stations are generally adding to the cost of diesel adds 5,500 miles to the break-even point for a new car buyer who chooses diesel instead of petrol. Diesel cars typically cost £1,500 more but the saving from better fuel efficiency should eventually recoup that."
AA president Edmund King said: "To be fair, there is often much greater variation in the price of diesel among retailers in a town than with petrol. However, on average, the profit margin on diesel is consistently at least a penny higher than with petrol. The clear message to diesel drivers is to take advantage of the greater range of prices locally. Some forecourts are more diesel-friendly than others."
Meanwhile, the AA highlighted HM Revenue and Customs' figures showing that UK petrol sales rose last month to 1.56 billion litres - a 49 million litre increase on the April 2013 figure. This followed record low figures for February and March this year.
Government tax receipts from petrol sales recovered to £904 million in May 2013 - up £28 million on the month before and £107 million higher than the year low in March.
Diesel sales, which include haulier and other commercial use, fell back from 2.29 billion litres in April to just under 2.26 billion in May. Overall, petrol and diesel sales in May were the highest so far this year at just under 3.82 billion litres, bringing in fuel duty receipts of more than £2.21 billion.