Before she set out, Ms McKessick had checked the media reports in case the new regulations applied also to buses. She was satisfied that they did not.

The driver and ticket collector at Aberdeen bus station had other ideas. Ms McKessick was not allowed to board the bus with any hand baggage. Her insistence that she was willing to be searched fell on deaf ears.

For Ms McKessick, 29, who was returning home to Dover after visiting relatives in Tarves, near Aberdeen, it was the beginning of a 12-hour nightmare. Separated from her travel survival kit, including food, drink and a good book, she found as the bus got under way that it had no running water and no heating, and that the lavatory was out of order.

Her heart sank further as the bus stopped to pick up passengers in Dundee and Glasgow, all of whom were allowed on with their full complement of luggage. By the time she arrived at Victoria station nearly 12 hours later, Ms McKessick felt ill, dehydrated and was nursing an overwhelming desire to make a formal complaint to the bus company.

“It was the worst journey of my life,” she said. I couldn’t even sleep through it; it was just totally chaotic.”

Megabus has apologised and says that it has begun an investigation.

The company is known to its regular passengers as the Ryanair of the M6. Fares for the journey of more than 500 bottom-numbing miles start at £1 — but even the most underfunded terrorist might balk at the hardship.

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