The Heathrow Express can no longer claim it runs "every 15 minutes" after a customer complained that the frequency drops to once every 30 minutes late in the day.

Heathrow Express said its core message of "15 minutes, every 15 minutes" was the service they offered to passengers "in line with overall airport flight patterns".

But it said the ongoing Crossrail engineering works were affecting the service, meaning that three trains in each direction were cancelled after 10pm from Monday to Thursday, leading to a half-hourly service during this time.

This was the equivalent of 2.2 per cent of trains affected across the week, and the amendment was displayed on the timetable section of the website and on printed timetables available in stations.

The Advertising Standards Authority (ASA) said consumers would understand "every 15 minutes" to mean that a Heathrow Express service would be available every 15 minutes throughout the whole day.

It said: "We understood that the service did not operate on a 24-hour basis, but only operated between 5am and midnight, and that it therefore was not available every 15 minutes. Furthermore, we understood that the service during post-10pm operating hours during the week was reduced to a half-hourly service.

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Heathrow express Photo: Alamy

"We appreciated that the service was affected by Crossrail engineering work and acknowledged Heathrow Express' statement that this accounted for only 2.2 per cent of their service, but noted that the reduction in service was regular and ongoing during this period of engineering work, and that it entailed the 15-minute service being unavailable during this time.

"We therefore considered that information regarding the hours of operation and the regular reduction in service should have been made clear to consumers as part of the claim itself."

It noted that the information was available on the timetable section of the website and on printed timetables, "but we considered that this was insufficiently prominent and also served to contradict the headline claim that the service operated every 15 minutes".

The ASA told Heathrow Express not to repeat the claim "every 15 minutes".

A spokeswoman for the Heathrow Express said: "We respect the decision made by the ASA and are now reviewing our advertising because we want to ensure we provide the best service possible for our customers.

"We are proud to offer passengers the quickest possible journey between central London and Heathrow but occasionally there are planned engineering works that may affect our service.

"We recommend passengers check online or on Twitter before they travel."

The ASA told Heathrow Express not to repeat the claim "every 15 minutes".

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A spokeswoman for the Heathrow Express said: "We respect the decision made by the ASA and are now reviewing our advertising because we want to ensure we provide the best service possible for our customers.

"We are proud to offer passengers the quickest possible journey between central London and Heathrow but occasionally there are planned engineering works that may affect our service.

"We recommend passengers check online or on Twitter before they travel."

Research published last year suggested that the Heathrow Express was the most expensive airport rail journey in Europe, with flights to Spain available for less.

Of the 47 airports in Europe included in the survey for Flightright.com, four of the five most expensive airport trains served London. Stansted was the second costliest to reach by rail – with tickets in 2014 priced at £23.40 one-way (or £33.20 return), followed by Stockholm (£22.07), Gatwick (£17.70) and Luton (£15.50).