A surge in the wholesale cost of petrol across Europe has already led to a rise in UK petrol and diesel prices, with more misery possibly to come, the AA said.
On average, UK petrol prices have risen from 134.61p a litre in mid-June to 135.78p now, while diesel has gone up from 139.16p a month ago to 140.24p now.
The AA said an increase in the cost of petrol across north west Europe, combined with a weaker pound, heralded a potential 5p increase in pump petrol costs.
It added that should petrol go up 5p a litre then a family from west London, for example, heading off on holiday in a typical family car to Cornwall would pay £2.90 more for the return trip than it would have done in June.
At present, London and the North West have the cheapest petrol, at 135.5p a litre on average.
Northern Ireland, although enjoying the smallest price rise over the past month, is still the most expensive region for petrol at 136.6p.