The BT Tower has been sold by the telecoms firm to a US hotel business for £275 million.
MCR Hotels will convert the 177-metre tall communications tower into an upmarket hotel and open it up to the public.
BT said the deal will preserve the future of the Grade II listed building, which was completed in 1964, becoming the tallest building in London at the time.
The tower initially had a revolving restaurant on its top floor which took 22 minutes to rotate and had expansive views across the city.
However, the viewing area was closed to the public in 1971 after a bomb exploded there.
Changes in the telecoms industry mean that many network operations previously provided by the tower are now completed through BT’s fixed and mobile networks, with its microwave aerials removed more than a decade ago.
Brent Mathews, property director at BT Group, said: “The BT Tower sits at the heart of London and we’ve been immensely proud to be the owners of this important landmark since 1984.
“It’s played a vital role in carrying the nation’s calls, messages and TV signals, but increasingly we’re delivering content and communication via other means.
“This deal with MCR will enable BT Tower to take on a new purpose, preserving this iconic building for decades to come.”
MCR Hotels owns around 150 hotels, including the historic TWA Hotel in New York City, designed by Finnish-American architect Eero Saarinen.
The hotel firm said BT will “take a number of years to vacate the premises” due to the complex equipment at the site, with payment for the deal to be made over multiple years.
It added that it will partner with the Camden-based Heatherwick Studio on designs for the proposed hotel.
Tyler Morse, chief executive and owner of MCR Hotels, said: “We are proud to preserve this beloved building and will work to develop proposals to tell its story as an iconic hotel, opening its doors for generations to enjoy.”