THE MEGA merger between Sainsbury and Asda is being probed over fears it could send petrol prices sky-high.

The Sun can reveal the Competition and Markets Authority (CMA) has pledged to study whether the £15 billion tie-up could lead to forecourt closures.

Sainsbury and Asda between them would be the biggest petrol retailer in the country with an 18 per cent market share – bigger than Tesco, BP or Shell.

Tory MP Rob Halfon last month warned the deal could mean the end of price wars and pleaded with the CMA to study whether motorists would miss out.

In a reply seen by the Sun, CMA chair Andrew Tyrie has assured his officials will ensure campaigners’ concerns are “taken into account”.

He says: “Our duty to promote competition for the benefit of consumers extends across all markets in the UK.

“Our investigation into the proposed merger will therefore cover all products and services in which the parties compete, including the supply of fuel.”

Petrol prices hit a four-year high earlier this week as oil prices spiral.

Speaking last night, Rob Halfon said: “Every way they drive motorists are being hammered with higher costs.

It is vital that the Asda-Sainsbury merge does not mean more misery for drivers.

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“It is why I wrote to the CMA and strongly welcome their investigation to ensure that prices at the pump are kept low.”

Petrol prices have climbed to a four-year high – as world oil prices spiral.

Chancellor Philip Hammond sparked fears of further pain for motorists on Tuesday by giving the biggest hint yet that the eight-year fuel duty freeze could be over.