BA’s transatlantic route is one of its most profitable but the airline said that passenger numbers last month, one of the industry’s busiest periods, fell from 590,000 to 580,000.

Analysts speculated that the passengers were lost to rivals, including Sir Richard Branson’s Virgin Atlantic, but BA did not give any reason for the decline.

One analyst said: “It’s difficult to argue that America as a holiday and business destination is less attractive now.”

The transatlantic decline was offset by an increase in premium travel during the month, as well as growth on BA’s UK and Europe, Asia-Pacific and African routes.

BA’s shares rose almost 5 per cent, or 11½p, to 242½p as investors applauded the improved figure for premium travel.

The company said that market conditions remained “broadly unchanged”.

Overall, 2.7 million passengers flew with BA last month, up 1.8 per cent on the previous corresponding period. In the first nine months of BA’s financial year, which ends on March 31, the passenger tally was 27.4 million, up 1.6 per cent on the same time last year.

The airline said that passenger capacity had been 3.9 per cent higher last month than in December 2003, but traffic, measured in revenue passenger kilometres (RPK), was up only 3.2 per cent.

The shortfall led to a 0.6 per cent drop in the passenger load factor to 72.7 per cent.

BA said that the increase in RPK was led by a 6.8 per cent surge in premium traffic, with non-premium take-up 2.7 per cent higher. It did achieve solid growth in passenger load factors — the percentage of seats sold — on its UK and Europe (up 1.2 per cent to 62.4 per cent) and Africa and Middle East (up 1.9 per cent to 76.7 per cent) routes. But load factors on its Asia-Pacific routes, alongside the transatlantic network — its most profitable segment — were down 1.3 per cent to 74.3 per cent. The US route experienced a 2 per cent drop in load factor to 75.2 per cent.

BA has flagged a £6 million non-cash accounting loss in the December quarter because of the weaker Japanese yen.

The improvement in premium travel is a rare piece of good news for BA, which has been dogged by high fuel prices and lower fares.

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