Kayak, the popular multi-airline airfare search engine, thinks Microsoft Bing’s new travel search engine looks so much like its own that it’s confusing Kayak users. The travel search company sent Microsoft a legal letter last week telling them to cut it out, Wired.com has learned.

Microsoft heralded its travel search as one of the key ways that its revamped search engine Bing bested Google by helping users make decisions, rather than just finding information.

Its search results for an itinerary presents users with sliders and check boxes on the left that let searchers change times and specify airlines. Search results reload instantly as boxes are clicked and sliders slid.

There’s no question Bing feels like Kayak. When Microsoft showed us the search engine under embargo, this reporter’s first comment upon seeing the travel page demo’d was “This looks like Kayak.” Our Bing review described its interface as “uncomfortably close to Kayak’s,” an observation that others made as well.

“We have contacted them through official channels about concerns about the similarities between Bing and Kayak,” Kayak’s chief marketing officer Robert Birge told Wired.com “From the look and feel of their travel product, they seem to agree with our approach to the market.”

That’s careful language for “Microsoft copied our stuff wholesale.”

Microsoft’s Whitney Burk denies that there’s any copying going on.

“We are discussing the matter with Kayak,” Burk said in emailed statement. “Bing Travel is based on independent development by Microsoft and Farecast.com, which Microsoft acquired in 2008. Any contrary allegations are without merit.”

Copyright law offers some protection for a website’s look-and-feel, but it’s not easy to prove.