LONDON (Reuters) - Budget airline easyJet <EZJ.L> said rising fuel costs sent first-half losses up threefold to 57.5 million pounds, but that summer bookings were ahead of last year.

The company, Europe's fourth biggest airline, reiterated an earlier warning that record fuel costs would erode full-year profit if they continued to rise, but that demand from its passengers remained strong.

"We are seeing no sign in reduction in discretionary air travel ... Maybe people are choosing airlines more selectively, but we are seeing no fall off in weekend breaks, no change in bookings," Chief Executive Andy Harrison told reporters on Wednesday.

He said the Luton-based airline was run more efficiently than many of its rivals, which he predicted would fail to survive in the current high cost environment and should not hope to be rescued by easyJet.

"It's hard to understand why we would want to buy weak airlines with old aircraft. We'd be happy to wave goodbye," Harrison said.

Shares in easyJet, which have halved during the year to date, were up 1.7 percent by 9:36 a.m. at 303 pence, valuing it at 1.3 billion pounds.

However, Collins Stewart (LSE: CLST.L - news) analyst Andrew Fitchie said the airline was not immune to an economic downturn.

"Consensus is only currently modelling the impact of increased fuel costs; a material economic slowdown is not being discounted and we believe this represents a further risk to the airlines and low cost carriers in particular," he said in a 'sell' note.

BIGGEST CHALLENGE

EasyJet (LSE: EZJ.L - news) reported a 15 percent growth in first half passenger numbers to 18.9 million, of which just 2.5 percent was attributed to last year's acquisition of GB Airways. The load factor, a measure of how much it fills planes, held steady at 81 percent.

Harrison said high fuel prices -- driven by the soaring price of oil -- remained the firm's biggest challenge.

"The price of jet fuel has risen 35 percent over the last three months and is now 80 percent higher than last year," he said.

If fuel averaged around $1000 (511 pounds) a tonne it would increase the company's bill by 45 million pounds in the second half, he said. It is currently around $1100-1150 a tonne.

The company, Europe's second biggest budget carrier in terms of passengers behind Ireland's Ryanair <RYA.I>, said first half revenues rose 24 percent to nearly 900 million pounds.

It added that it had successfully integrated GB Airways, the domestic rival it bought in October.

(Editing by Stephen Weeks)