It was a promise bound to appeal to travellers: Ryanair will get you from London to Brussels faster and cheaper than the Eurostar.

The only problem, the advertising watchdog ruled yesterday, was that it was not true.

The budget airline has been banned from claiming that it was quicker and better value to take a plane rather than the train to the Belgian capital, after the Advertising Standards Authority upheld complaints that the promotion was misleading.

The decision prompted an angry response from Ryanair. It branded the ASA “Absolutely Stupid Asses” who “can’t add and, worse still, can’t subtract”.

As part of a press campaign, Ryanair compared its flight, taking one hour and ten minutes, to the train trip lasting two hours and 11 minutes. The airline claimed to offer a better deal than its competitor: “Ryanair one way — from £15 — taxes and charges included. Eurostar one way — from £27 — taxes and charges included”.

Its flights were 44 per cent cheaper and also more punctual, it said, with 89 per cent of flights on time compared with 83 per cent of Eurostar trips. However, the ASA found that the claims misled customers because they failed to take into account that travelling from London and Brussels city centres to the two airports used by Ryanair would add one hour and 45 minutes to the journey time.

Stansted airport is about 25 miles (40km) from London, while Charleroi is about 28.5 miles (46km) from Brussels. The watchdog upheld complaints by the Eurostar Group and the public, ruling that the advert breached advertising rules relating to substantiation, truthfulness and comparisons with identified competitors.

It also emerged that the airline had taken Eurostar’s punctuality figure from an article on the BBC website that was more than two years old.

A statement from the airline said: “Passengers booking Ryanair today to travel from London to Brussels enjoy fares of just £10 one way including taxes. Eurostar is charging 15 times more (£154 one way). No amount of stupid rulings from the ASA quango will hide the fact that 50 million passengers will save over £5 billion by flying with Ryanair this year. Only the very rich or the very slow waste their time and money on Eurostar.”

When The Times visited the Ryanair and Eurostar websites yesterday to book a return journey to Brussels this weekend the most expensive non-flexible fare on Ryanair was £157.89. The most expensive Eurostar ticket was £144. The cheapest Ryanair flight, leaving at 6.40am on Friday, was £54.89. The cheapest Eurostar journey was £125.19.

The airline had argued that its advert made clear that its flights went from Stansted. It also said that travellers would have to pay to get to Eurostar railway stations, just as they would have to pay to get to airports.

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