Nearly 14 million people tuned in to watch comedy legend David Letterman broadcast his final "Late Show" on American television, the program's largest audience in 21 years, CBS said Thursday.

Wednesday's show averaged 13.76 million viewers, its largest audience since February 1994, when it followed CBS's primetime coverage of the Winter Olympics in Norway, the television network said.

There was a knock-on effect for British newcomer James Corden, host of "The Late Late Show," who averaged four million viewers directly after Letterman signed off for the last time.

CBS said it was the show's largest audience with any host. Corden, a major star in Britain but previously little known in America, took over the Los Angeles-based show in late March.

Letterman's retirement aged 68 marks the end of a cultural institution in America and an extraordinary 33-year career.

His emotional final show, packed with jokes and farewells from US presidents, Hollywood stars and devoted fans, was praised by critics.