An “urgent” investigation into petrol station operators has been launched over fears companies are pocketing the cut to fuel duty aimed at keeping petrol prices down.

In a letter to the Competition and Markets Authority (CMA), Business Secretary Kwasi Kwarteng wrote that people were “rightly frustrated” that the 5p-a-litre reduction had not stopped prices from soaring.

Noting regional disparities in petrol prices, the Spelthorne MP asked for recommendations to “strengthen competition” in the petrol market by early July.

Mr Kwarteng said: “Drivers should be getting a fair deal for fuel across the UK.

Bucks Free Press: Kwasi Kwarteng (PA)Kwasi Kwarteng (PA)

“The British people are rightly frustrated that the £5 billion package does not always appear to have been passed through to forecourt prices.

“I am writing to you to ask that the CMA conduct an urgent review of the fuel market … to explore whether the retail fuel market has adversely affected consumer interests.”

The competition watchdog has previously made Asda and Morrisons sell off a number of forecourts during private equity acquisitions.

Last Tuesday, data firm Experian Catalist revealed the average price of petrol at the pump had reached a record 180.7p.

Jack Cousens, the AA’s head of roads policy, welcomed Mr Kwarteng’s move but called for “more urgent action”.

“To relieve pressure at the pumps we need an immediate 10p cut to fuel duty,” he argued.

Bucks Free Press:

“Longer term, the CMA should consider extending the pump price transparency available in Northern Ireland to the rest of the UK.

“The Consumer Council’s fuel price checker stimulates competition and has led to drivers there enjoying the lowest fuel prices in the UK.”

It comes as the cost of filling up a family car has exceeded £100 for the first time ever.

RAC fuel spokesman Simon Williams said the average price of petrol crossing the “thoroughly depressing threshold of £100 a tank” meant it was “a truly dark day” for drivers.

He went on: “There’s almost certainly going to be upward inflationary pressure, which is bad news for everybody.

“While fuel prices have been setting new records on a daily basis, households up and down the country may never have expected to see the cost of filling an average-sized family car reach three figures.”