Experts blame commodity market speculators, rising oil prices and a weak pound for the likely surge.

The cost of Brent crude has risen from $100 to $109 since the last week of June and the pound has fallen from $1.56 in mid-June to $1.51 now.

The AA said fuel stations in many areas have wasted ‘little time’ in passing on the higher cost.

Average UK petrol prices are up from 134.61p a litre in mid-June to 135.78p.

Edmund King, the AA’s president, said: “After the price of petrol stabilised at around 134.6p a litre through much of this June, and weeks were filled with beautiful weather and sporting excellence, it was perhaps inevitable that oil and fuel market speculators would cast a black cloud over what was promising to be a glorious summer.”

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Should the full 5p-a-litre potential increase in petrol prices hit UK forecourts, a family from Hounslow heading off on holiday in a typical family car to Plymouth will pay £2.90 more for the return trip than it would have done in June.

Elsewhere regionally, London, joined by the North West, has resumed its position as the cheapest part of the UK for petrol at 135.5p a litre. Northern Ireland, although enjoying the smallest price rise over the past month, is still the most expensive region for petrol at 136.6p.

Scotland and East Anglia share the position of most expensive areas in the UK for diesel, both averaging 140.8p a litre, while the North West again bags the cheapest spot with 139.7p