Mystery shoppers revealed drivers pay 117 per cent more on average for food and beverages at service stations compared to supermarkets, with a bottle of water costing as much as four times more.

Beaconsfield services on the M40 was the worst offender, charging £2.29 for a 500ml bottle of water, £1.85 more than the average supermarket price.

Water was revealed to be the most overpriced purchase despite being available for free in most service stations.

The research by insurer Admiral looked at 21 motorway services stations across the UK.

Durham services on the A1(M) was home to the most expensive basket of sweets and savoury snacks and drinks at £16.21, almost £10 more than the average supermarket price of £6.89.

Lancaster (Forton) on the M6 and Woolley Edge on the M1 ranked as the second most expensive services, both coming in at £16.17 for the same items.

Researchers also found the average price of fuel at motorway services was almost 14 per cent higher than the national average.

Prices varied considerably between service stations with a cost of a litre of unleaded ranging from 149.9p to 129.9p.

Clare Egan, head of motor product at Admiral, said: “We all expect to pay a little more for convenience, but the amount people are charged at service stations amounts to highway robbery in some cases.

“Many of us will be making cross-country trips with our families over the coming weeks, and it will be tempting to stop en route for a snack or petrol. By planning ahead, you could avoid the hyper inflated service station costs save some money for Christmas and help the environment at the same time.”

Top tips to save money on a long car journey

Plan ahead and buy food in from the supermarket before you set off, or even better take a packed lunch and refillable water bottle from home.

Use an app such as refill.org.uk to identify places to refill water for free.

Fill up on fuel before you get onto the motorway to ensure you aren’t paying inflated prices.

Bring some activities and snacks for the children so you are prepared should they want to stop off to avoid expensive treats.

Consider pulling off the motorway to a nearby supermarket if you need to re-stock: prices are considerably lower.

Don’t forget to make sure you carry out all the appropriate vehicle checks before you start a long journey – checking tyre pressure and water levels will mean you travel safely and don’t have the unexpected cost of a broken-down car.