New research from What Car? discovered a WH Smith motorway services outlet that was charging consumers a 91% motorway mark-up over its high street outlets on a 750 ml bottle of water – more than Harvey Nichols in Knightsbridge.

M&S also charges inflated prices in its motorway shops, including a 25% increase on a bottle of water, 16% on a sandwich and 15% on a large sausage roll. Even fast food outlet Burger King charges 12% more on the motorway than it does in its high street restaurants.

Fuel is more expensive, too, with motorists having to pay an average motorway levy of 5.2% for unleaded fuel and 3.4% for diesel. Consumers filling their fuel tank at motorway services will pay on average £77.00 more per year than they would if they filled up off the motorway.

What Car? editor-in-chief Steve Fowler said: “It is unforgiveable for retailers to take advantage of motorists in such a brazen manner. With fuel prices at an all-time high, the 5.2% average motorway fuel mark-up is hard for consumers to fund, and a 91% mark-up on a bottle of water is utterly unacceptable. We suggest that all motorway users plan their journeys carefully and try to avoid using such greedy retailers as much as possible.”