Phil Lesh, bassist and co-founder of the American rock group The Grateful Dead, has died at the age of 84.

The musician’s official Instagram account states that he “passed away peacefully this morning.” He was surrounded by his family.

The psychedelic band, which formed in California in 1965, broke up thirty years later after the death of frontman Jerry Garcia.

Lesh was with them from the start – also joining the group’s other surviving members for a US reunion tour in 2003 and a final series of concerts in 2015.

Lesh’s Instagram account said he “brought immense joy to everyone around him and left a legacy of music and love. We ask that you respect the Lesh family’s privacy at this time.”

With a signature trippy mix of rock, folk and jazz, The Grateful Dead are perhaps one of the most influential bands in American history and wrote the soundtrack for the countercultural generation of the 1960s.

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

Over 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 bond with its audience.

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

Loyal fans known as “Deadheads” often followed the band from city to city across the US to hear them play at packed concerts.

The band has always made things easy for its fans recording her concerts and distributing tapes to their colleagues around the world.

Despite their huge following, they only scored one top 10 hit in the US in 1987 with Touch of Gray.

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

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

He also underwent a liver transplant in 1998 and became a passionate advocate for organ donations.

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

Additional reporting by David Bamford.