The transport secretary has called for an investigation into “exploitative” motorway fuel prices.
Chris Grayling has told the competitions watchdog some operators charged up to 19p more per litre than forecourts in towns or cities.
He said the UK’s three biggest operators – Moto, Welcome Break and RoadChef – should be investigated to ensure drivers got a “fairer deal”.
But Roadchef said it did not set the prices of fuel at its sites.
And Moto said the costs of building and operating the roadside services were behind high prices.
It comes as millions of Britons prepare to use the motorways over Easter.
Writing to Andrea Coscelli, head of the Competition and Markets Authority (formerly the Office of Fair Trading), Mr Grayling requests that “the CMA consider opening an investigation into the retail price at motorway service areas (MSAs)”.
“I am concerned that prices which are higher than other forecourts may exploit users in a situation where there is less choice and competition, and discourage motorists from stopping and re-fuelling when, for safety reasons, they should.”
He added: “I would welcome a view from the CMA on whether the three private companies that currently operate the majority of MSAs are exercising market power to the detriment of motorists.”
Moto, Welcome Break and RoadChef run most of the 112 motorway service areas across the UK.
According to industry figures published on Thursday, the average price for unleaded petrol at MSAs is 137.7p per litre, compared to a UK average of 120.11p per litre.
The average unleaded price at supermarkets is just 116.74p.
It means filling up the average car with unleaded fuel costs around £10 more on the motorway.
Mr Grayling pointed to RAC figures from 2011, which showed that average petrol prices were just 7.5p higher, showing the “situation had worsened”.
He said he understood that MSAs had higher overheads and infrastructure costs, but that their higher prices were “not fully explained”.
In 2013, a study of the UK petrol and diesel market found that fuel was significantly more expensive at motorway service stations.
The Office of Fair Trading was concerned that motorists were not able to see the prices until they had left the motorway and requested new signs on motorways to display prices.
The plans were scrapped after a trial-run of the signs produced no evidence of savings.
Simon Williams, fuel spokesman for the RAC, welcomed the minister’s intervention as “great news for motorists”.
“We have long called for something to be done about the cost of motorway fuel because there’s nothing to justify the sky-high price.
“Drivers filling up motorway services often put in the bare minimum, but this can easily backfire. Running out of fuel on a motorway puts lives at risk.”
The AA said motorists had too long been treated like “hostages” by MSAs. “We welcome any move to deter rip-off motorway pump prices,” the organisation said.
A spokesperson for Roadchef commented: “Roadchef does not operate the petrol station forecourts at any of our service areas and does not set the price of fuel.”