The tallest building in the United States now bears the name of a Bermuda-domiciled company after Chicago's Sears Tower was renamed the Willis Tower.

In getting its name on the building Willis Group Holdings Ltd. - which is based in Bermuda, but has its largest operations in London and New York - is putting one over its insurance broker rival Aon Corp.

The Willis Tower, 315 feet high and with 108 floors, is taller than the Aon Center, the 83-floor edifice about a mile away, which serves as Aon's headquarters and is Chicago's second tallest building.

Willis's naming rights for the iconic skyscraper will run for 15 years, as part of its lease of 140,000 square feet on three floors of the building.

The move will allow the company to bring together around 500 employees who were located in various Chicago area offices.

"We compete with Aon quite a bit," Don Bailey, chief executive officer of Willis's North American business, told Bloomberg News. "But the size of buildings is not really part of the terms of competition. So, we'll stick with the clients."

Chicago Mayor Richard Daley unveiled the tower's new name on a large black sign in the lobby with the help of Joseph Plumeri, the chairman and chief executive officer of Willis, in a ceremony held last Thursday.

"We believe in Chicago," Mr. Plumeri said. "You will find over time that Willis is not going to just have its name on the building, it's going to have an impact in society, in the community."

Mr. Plumeri said the company plans to bring hundreds of jobs to the city and help in the community. He presented a cheque for $100,000 to the volunteer organisation Chicago Cares, said his company's 500 Chicago employees would help out the group and pledged to donate another $100,000 to Chicago 2016, the city's bid for that summer's Olympic Games.

Sears Tower was designed for Sears, Roebuck & Co. and opened in 1973, Willis said. The retailer, which became Sears Holdings after a 2005 combination with Kmart Holdings Co., sold the skyscraper in 1994 and vacated it the next year.

Renaming an icon is often unpopular and already there are rumblings among the locals in the Windy City.

A website has sprung up bearing the address, www.itsthesearstower.com. The site urges browsers to sign up to a campaign to persuade Willis to allow the tower to revert to its former name.

The letter, addressed to Willis, that it asks people to put their names to states: "Changing the name of the Sears Tower to promote your business will tarnish your company's reputation in the hearts and minds of people worldwide. Especially in Chicago, the name Willis Tower will remind people of what you have taken away, rather than what you have brought to the city.

"Please return the Sears "Please return the Sears Tower name to the building, and explore other ways to promote your business that will preserve the history and heritage of Chicago."

Reluctance to let go of the name is understandable, Mr. Plumeri told the Associated Press. But, he added: "By the same token life moves on, nothing ever stays the same. Chicago is an evolving city."