MyTravel has been warned the merger with Thomas Cook could lead to it falling behind on the Internet.

For the past 18 months MyTravel has been improving its sites, optimising them for search engines and investing heavily in affiliate marketing.

This has seen it jump to the fourth most-visited online travel firm in January, according to Hitwise. Thomas Cook was sixth.

Combined, the MyTravel and Thomas Cook sites would have put it on a par with industry leader Expedia and ahead of Thomson and

The enlarged group, which currently sells around a quarter of its 6.3 million capacity on the web, could become a real force online.

However, online travel experts have warned consolidation could see the sites fall behind in the short term.

Ebookers managing director Alan Joseph said: “Thomas Cook may struggle with the innovation side as it looks to consolidate its technology.”

Travel search site Sidestep European managing director Kevin Eyres agreed.

“Integration will take a lot of time.”

Thomson Northern Europe new media director Graham Donoghue predicted the enlarged group will see online hits reduce by around 20% as its integrates its sites.

“Customers shop around and will currently be going on to both sites,” he said.