The day courtroom 56 swayed to the sound of 'A Whiter Shade of Pale'

It was the signature tune of the 1967 summer of love which went on to sell 10 million records worldwide.

Now nearly 40 years after the release of Procol Harum's "A Whiter Shade of Pale", the High Court in London is to decide who should be credited for composing the song.

Yesterday the song's evocative melody and unfathomable lyric filled courtroom 56 as Matthew Fisher, the band's original organist, pressed home his claim that his contribution entitled him to half the copyright and earnings, estimated to be £1m.

Mr Justice Blackburne was told by Mr Fisher's barrister that the musician had played a crucial part by composing an organ solo which introduces the song and is later repeated.

Iain Purvis QC told the judge: "We are dealing with one of the most successful pop songs ever written by British artists."

He added: "Mr Fisher now seeks a declaration that he is entitled to a share - an equal share in the musical copyright for the song as originally recorded."

Mr Fisher, now a computer programmer in Croydon, south London, who was a classically-trained musician, says he also made chord changes to the original sequence and added to the work with a counterpoint to the song melody.

The claim is being vigorously defended by Gary Brooker, the band's vocalist, who says he and lyricist Keith Reid, wrote the song. Mr Brooker based his tune on Johann Sebastian Bach's "Air on a G String" - which he had heard on a Hamlet cigar advertisement - and Bach's Sleepers Awake.

Mr Brooker will argue that it is "extraordinary" that Mr Fisher, who left the band in 1969, is bringing the case after nearly 40 years.

But Mr Purvis said the suggestion that Mr Fisher had delayed for 39 years before stating his claim was "simply not true". Documentary evidence showed that, from as early as 1973, Mr Fisher had pointed out in interviews with journals including Melody Maker and New Musical Express that he had never been credited for his organ part contribution to the song.

Mr Purvis said Mr Fisher had not gone to court at the time because he "did not want to rock the boat".

"He was trying to make his way in the recording world. He did not want to make a big fuss and did not know whether he had a legal claim at all," said Mr Purvis.

"No one could have predicted that 'A Whiter Shade Of Pale' would still be being broadcast, played on juke boxes and used as ring tones in 2006."

The case continues.