Former Grateful Dead guitarist Bob Weir has sparked fears for his health after cancelling his remaining 2014 Ratdog shows.

The rocker has cited unspecified "circumstances" as the reason for his band's hiatus.

The news affects his U.S. tour, which was scheduled to begin in Boston, Massachusetts later this week (beg11Aug14).

Weir cancelled a string of shows last year (13) following a collapse at a gig in New York.

The veteran rocker was performing with his other group Further in Port Chester at the end of April that year (13), when he fell to the floor in the middle of a song, and then exited the stage with what bandmate Phil Lesh called a "strained shoulder".

Weir was back onstage with the group in Atlantic City, New Jersey on 27 April (13), but an ongoing mystery health issue forced him to scrap a series of appearances and shows.

He returned to the stage in June, 2013 to tape a new episode of his weekly web series, Weir Here, which was put on hold following his stage collapse, in front of a live audience at Terrapin Crossroads in San Rafael, California.