Average petrol and diesel prices have fallen but the situation remains fragile, according to the AA.
The average price of petrol on UK forecourts has fallen from 136.89p a litre in mid April to 133.35p in mid May. It had hit a year high of just over 140p on March 4, having started the year at around 132p.
Diesel, on average, now costs 138.17p a litre, down from 141.76p a month ago. It also reached a year high on March 4 of almost 146.5p a litre, having started the year at around 140p.
But the AA said that this week's International Energy Agency (IEA) warning that traders and speculators were taking control of the European fuel market meant that more and ups and downs in prices could be expected.
The drop in pump prices in the last few weeks follows a series of supermarket price reductions, led by competitive independents and other non-supermarket retailers, which have now reduced the cost of petrol to as low as 128p/129p on the east side of London.
The AA said: "However, wholesale price movements indicate the fragility of the price falls as petrol costs have rebounded, fallen away again and then bounced back over the past three weeks."
Averaging 132.8p a litre, Yorkshire and Humberside has resumed its position as the cheapest region in the UK for petrol. Northern Ireland is the most expensive for petrol at 135.1p and also for diesel, at 139.3p a litre. This is 2p more than the cheapest diesel area - Yorkshire and Humberside at 137.3p
AA public affairs head Paul Watters said: "Three times in the past 12 months, drivers have been hammered by £4-£5 hikes in the cost of a tank of petrol. It is clear that, if petrol and diesel wholesale price movements were transparent, families and businesses would have 10 to 14 days' notice of the next price shock - and hopefully the reason for it."
He went on: "The IEA report suggests that fuel prices will be increasingly subjected to speculation on the part of those who look for inflated short-term profit rather than those players looking to strike a balance between fair profit and supply and demand - without destroying their consumer market, refining capacity and the UK economy.
"We have seen petrol consumption in the UK fall to record lows this year. As many as 69% of AA members are cutting back on car use, other spending or both, and 86% of US drivers are using their cars less. The warning signs couldn't be any clearer."