The spiralling cost of business, first-class and supersonic air fares to the United States has led travellers to fly from the Continent instead of Heathrow.

Last year, supersonic transatlantic fares increased by nine per cent, first-class by 5.1 per cent and business class by 4.9 per cent, meaning it is more expensive to fly from London to the US than from any Continental city.

Fares on economy class increased by 3.2 per cent, and the American Express European Corporate Travel Index said premium rate fares would continue to rise.

It found a return business class fare from London to New York was 58 per cent more expensive than from Frankfurt, 53 per cent more than from Amsterdam, and 30 per cent more than from Paris.

Business class travellers from the UK were saving over a third (37.5 per cent) of the cost of a ticket by flying via Amsterdam, instead of direct from London to Chicago and 43.2 per cent to San Francisco.

Director of consulting services Matthew Davis said: "Despite weak economies and lower demand, with immunity from the competition of low-cost carriers on long- haul routes, traditional airlines have posted fare increases.

"With few airlines operating on these high-demand, premium routes, and with the open skies negotiation between the UK and US stalling, continued fare growth is likely in the foreseeable future.

"While corporations acknowledge the importance of travel and face-to-face meetings for business, travel managers are faced with spiralling costs.

"Corporations are under pressure to get the most mileage for their money."

As well as flying long- haul from a continental hub, the research found companies were choosing less expensive, but more restricted, long-haul business and first class tickets.

For travel within Europe, British businesses were switching to low-cost airlines, economy class or travelling by train.

Budget airline easyJet's takeover of rival Go has been cleared for take off by the Office of Fair Trading.

The tie-up should now be completed by the end of this month, although passengers will not see any difference until next spring.

Regulatory clearance in the Czech Republic and Portugal, where Go currently flies, is one of the few outstanding hurdles.

EasyJet has already completed a rights issue to raise £275 million towards funding the £374 million deal and won the support of shareholders.