Phil Lesh, bassist and co-founder of the US rock group The Grateful Dead, has died aged 84.

The musician's official Instagram account said he "passed peacefully this morning". He was surrounded by his family.

The psychedelic band, which formed in California in 1965, split 30 years later following the death of frontman Jerry Garcia.

Lesh was with them from the beginning - and also joined the group's other surviving members for reunion US tour in 2003 and a final series of concerts in 2015.

Lesh's Instagram account said that he "brought immense joy to everyone around him and leaves behind a legacy of music and love. We request that you respect the Lesh family’s privacy at this time."

With a distinctive trippy blend of rock, folk, and jazz, The Grateful Dead are arguably one of the most influential bands in American history, and wrote the soundtrack for the countercultural generation of the sixties.

Lesh was born in Berkeley, California, in 1940. He started out as a violin player before switching to trumpet, and later to bass guitar when he joined The Grateful Dead in 1965.

For the next three decades his improvisational skills complemented the melodies of lead guitarist Jerry Garcia and bandmates Bob Weir, Bill Kreutzman and Ron "Pigpen" McKernan.

He was best known for the song Unbroken Chain, about the band's connection with its audience.

Lesh also sang the wistful Box Of Rain, which he wrote while his father was dying.

Loyal fans, known as “Deadheads”, would often follow the band from city to city across the US to hear them play at packed-out concerts.

The band always made it easy for its fans to record its concerts and distribute tapes to their peers around the world.

Despite their massive following, they notched up only one top 10 hit in the US with Touch of Grey in 1987.

Although the cause of Lesh’s death is unknown, he had a series of health issues over the years.

In 2015, he announced he was being treated for bladder cancer in the US. Nine years before that he had surgery for prostate cancer and made a full recovery.

He also underwent a liver transplant in 1998, becoming a passionate advocate of organ donations.

Lesh is survived by his wife, Jill, and their two sons.

Additional reporting by David Bamford.