Entwistle, 57, died on Thursday, a day before the group was scheduled to begin a concert tour in the United States.

The Ox has left the building - we've lost another great friend. Thanks for your support and love. Pete and Roger

Tribute on Pete Townshend's website

It is believed he may have suffered a heart attack, although the cause of death will not be confirmed until after an autopsy is carried out on Friday.

Las Vegas police said there was no reason to believe there was anything suspicious about the death.

Fans are also deeply shocked by his death, and The Who manager Bill Curbishley added that Roger Daltrey and Pete Townshend were "devastated".

"The entire Who family is terribly saddened by John's passing," he said.

"Our deepest sympathies go out to his family, friends and the millions of Who fans the world over."

Bearded Entwistle, affectionately known as "Ox," died at the Hard Rock Hotel and Casino, said coroner Ron Flud, of the Clark County Coroner's office in Las Vegas.

Tears

Entwistle and Townshend: Friends for more than 40 years

Also a talented artist, Entwistle was due to open an art show featuring his work at the Grammy's Art of Music Gallery in Las Vegas on Friday.

The gallery's assistant manager Diana Tabor said staff and fans were in a state of shock.

"I'm emotionally distraught just now, it's just devastating," she said.

"I just had to break the news to a client who broke down in tears."

The Who were one of the biggest rock bands of the 1960s and 70s, with hits including the massive My Generation, Pinball Wizard and Substitute.

Backing vocals

Their rock musical Tommy, which stared a young Elton John as well as The Who front man Roger Daltrey and Entwistle playing himself, became a huge hit in the West End.

Entwistle has been described as one of the most influential musicians of his time

Entwistle's death leaves just two of the original members of The Who - singer Daltrey and guitarist/songwriter Townsend.

Original drummer Keith Moon died of an accidental overdose in 1978.

In addition to playing bass, Entwistle helped out on backing vocals.

His forays into song-writing were limited to album tracks and B-sides. He later released half a dozen solo albums.

Mods

Entwistle, born in the London suburb of Chiswick in 1944, joined Daltrey in a forerunner of The Who in the early 1960s while still working as a tax clerk.

The band really took shape in 1964 and its stylised "Mod" outfits and haircuts spawned a generation of look-a-likes.

Entwistle in 1975 when the band were at the forefront of the rock industry

They were among the original bad boys of rock with a habit of trashing stage equipment after each set.

With Entwistle, The Who continued to tour in recent years.

Two years ago on a tour they raised more than �1m for a teenage cancer charity.

The reverence given to the group by contemporary rock stars was proved when they were joined on stage for their last gig by Paul Weller, Noel Gallagher from Oasis and Bryan Adams.

The planned US tour was due to take in venues in various states in the US, including New York and California.

It is not yet known if the tour will continue without the bassist.

Entwistle was married twice, and has a son, Christopher, from his first marriage.