They are 5p per litre dearer than expected levels, the ­competition watchdog says.

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Bosses of big retailers will be formally quizzed about why costs have gone beyond average margins found in 2019.

The Competition and Markets Authority will also investigate if any failure in competition is leaving consumers paying higher grocery prices than they should be.

But internal documents from one supermarket indicate it has “significantly increased” its margin targets on fuel, prompting similar tactics across the sector.

Some outlets were also accused of being slow to provide information after several rounds of evidence gathering.

The CMA inquiry has led to further calls for a Pumpwatch system to be implemented to keep a check on fuel prices.

FairFuelUK said it was glad the CMA is “now showing some real teeth and demanding answers”.

• GARAGE owner Vijay Jaganathan claims to be selling Britain’s cheapest fuel at £1.29 for both petrol and diesel in Narberth, Pembrokeshire.