Research by the AA shows the petrol price gap between the big four supermarkets – Tesco, Asda, Sainsbury’s and Morrisons – and non-supermarket rivals has fallen to below 3p a litre for the first time in a year.

And it found both petrol and diesel were cheaper at Jet than at Sainsbury’s this month.

This week the UK’s average petrol price reached 120.11p a litre, up 0.63p on mid-January’s 119.48p.

Diesel is now at 122.32p a litre, a third of a penny more expensive than this time last month when the cost averaged 121.98p.

Compared to a year ago, filling a typical petrol tank costs £9.99 more, the AA said.

That means a family with two petrol cars is now spending £240.22 each month, compared to £203.90 in February 2016.

Edmund King, the AA’s president, said: “UK average petrol prices have hit a plateau of around 120p a litre for more than a fortnight, the perfect setting for local price variety and shopping around. Savings can be as much as £4-£5 a tank on some routes and drivers using fuel price apps, such as the AA App, will be discovering the value of searching for better deals.

“We accept that it is the prerogative of any retailer to charge what they like for fuel and, in the past, we have seen supermarkets switch to fighting their price wars in the aisles rather than on the forecourts. However, this latest change of tactic comes after nearly 15 months of tight fuel-price competition among supermarkets.

“Drivers need to keep their ears and eyes open to locate lower pump prices. That may be a case of buying fuel in a neighbouring town with more competitive prices.”

He said along the A3, from Portsmouth towards London, supermarket petrol varies by as much as 8p a litre, a difference once associated with the gap between motorway and non-motorway prices.