Nikolaus Harnoncourt, one of the most highly regarded classical music conductors of recent times and a pioneer in early music, has announced his retirement.

"My bodily strength requires me to cancel my future plans," the Austrian said in a hand-written farewell letter to the audience of the hallowed Musikverein concert hall in Vienna on Saturday.

Harnoncourt, who turned on 86 on Sunday, had already cancelled, due to ill health, plans to conduct two concerts at the Musikverein by the Concentus Musicus, the ensemble he created in 1953.

"An unbelievably deep relationship has developed between us on the stage and you in the hall -- we have become a happy community of pioneers!", Harnoncourt wrote in the letter, printed in the Musikverein concert programme.

Count Nikolaus de la Fontaine und d'Harnoncourt-Unverzagt was born in Berlin in 1929 to a granddaughter of a Habsburg Archduke and an Austrian count. He grew up in Graz, southern Austria.

Training as a cellist, his intensive research into historical instruments and period performance practice led him to set up the Concentus Musicus, which began giving concerts in 1957.

Organised by the musicians themselves, with help from their wives and partners, they specialised in renaissance and baroque music as well as classical works by the likes of Bach, Beethoven and Haydn.

in 1969 he quit the Vienna Symphony Orchestra after his insistent questioning in the stuffy Vienna of the time raised hackles as it ran contrary to the norms of the established classical music scene.

Conducting both orchestral works and operas, Harnoncourt's ideas gained wider currency, and now even the world's greatest modern-instrument orchestras like the Vienna and Berlin Philharmonics use key elements of period practice.