Ray Thomas performing with the Moody Blues in Las Vegas in 2001. He co-founded the British rock group in 1964. (Ethan Miller/Reuters)

Ray Thomas, a British flutist and singer-songwriter who co-founded the Moody Blues, whose psychedelic mix of synthesizers, guitars and classically inspired orchestral arrangements helped establish the genre known as progressive rock in the late 1960s, died Jan. 4 at his home in Surrey, south of London. He was 76 and was scheduled to be inducted into the Rock and Roll Hall of Fame with his bandmates in April.

His music labels, Cherry Red Records and Esoteric Recordings, announced the death but did not disclose the cause. Mr. Thomas said in 2014 that he had been diagnosed with prostate cancer.

Mr. Thomas performed in rock and blues bands in the English city of Birmingham before founding the Moody Blues in 1964 with fellow musicians including keyboardist Mike Pinder and guitarist and singer Denny Laine.

The band's roots lay in blues and R&B, but its 1964 cover of Bessie Banks's "Go Now" was a foretaste of the lush, orchestral sound that came to be called progressive rock.

Their 1967 album "Days of Future Passed" is considered a prog-rock landmark, and featured a signature flute solo by Mr. Thomas on the single "Nights in White Satin." The album was credited as a model for popular 1970s groups such as Yes, Genesis and Supertramp.

Raymond Thomas was born in Stourport-on-Severn, near Birmingham, on Dec. 29, 1941. He sang in a choir as a teenager and was inspired to learn the flute from a grandfather who played the instrument, according to the website All Music.

The Moody Blues in 2001, featuring Mr. Thomas alongside guitarist Justin Hayward, bassist John Lodge and drummer Graeme Edge. (Ethan Miller/Reuters)

Mr. Thomas wrote several songs for the Moody Blues, including the trippy "Legend of a Mind" and "Veteran Cosmic Rocker," and released two solo albums after the band broke up in 1974. The Moody Blues later reformed, and Mr. Thomas remained a member before leaving in 2002 because of poor health.

He married Lee Lightle in 2009. A complete list of survivors was not immediately available.