The first notes in the longest and slowest piece of music in history, designed to go on for 639 years, are being played on a German church organ on Wednesday.

The three notes, which will last for a year-and-a-half, are just the start of the piece, called As Slow As Possible.

Composed by late avant-garde composer John Cage, who died in 1992, the performance has already been going for 17 months - although all that has been heard so far is the sound of the organ's bellows being inflated.

We started discussing - what is as slow as possible for the organ? Hans-Ola Ericsson Composer

The music will be played in Halberstadt, a small town renowned for its ancient organs in central Germany.

It was originally a 20-minute piece for piano, but a group of musicians and philosophers decided to take the title literally and work out how long the longest possible piece of music could last.

They settled on 639 years because the Halberstadt organ was 639 years old in the year 2000.

"We started discussing - what is as slow as possible for the organ?" Swedish composer and organist Hans-Ola Ericsson told BBC Radio 4's Today programme.

"We, a group of theologians, musicologists, philosophers, composers and organists, met during a couple of years solely to discuss this question. It was rather wonderful to have one topic to discuss at length."

Hopefully the aesthetics and the ideas of John Cage will manage to survive Hans-Ola Ericsson

"We came up with the answer that the piece could last for the duration of the organ - that is the lifetime of an organ."

Mr Ericsson said John Cage would have liked what they had done with it.

"It's a sound that we give to the future to take care of, and hopefully the aesthetics and the ideas of John Cage will manage to survive."

The first note is due to be struck at 1800 local time (1700 GMT) on Wednesday.

The performance follows a legal case in which composer Mike Batt was forced to pay a six-figure sum to Cage's publishers, who accused him of plagiarising a silent piece of music.