A High Court judge ruled that Matthew Fisher, 60, who played the distinctive Hammond organ parts that made the song a worldwide hit, is entitled to a 40 per cent share of the musical copyright.
Having written the song, they hired Mr Fisher, then a young musician, to play on the Bach-inspired track.
It was his creative contribution as a composer rather than just a performer that made the song a hit, Mr Fisher argued.
Mr Justice Blackburne endorsed this, awarding him a co-authorship credit and a share of future royalties.
Mr Fisher will be entitled to 20 per cent of royalties backdated to May 2005 when he made the claim, while the 50 per cent cut that Mr Brooker has enjoyed for 40 years will be reduced to 30 per cent.
It is believed that Mr Fisher will receive a five-figure sum.
Mr Brooker, 61, who was unable to attend the hearing because he had just returned from tour, said in a statement that it was a dark day for the music industry.
He said: “Any musician who has ever played on any recording in the last 40 years may now have a potential claim to joint authorship.
“It is effectively open season on the songwriter. Songwriters and publishers will now have to view all musicians with suspicion as potential claimants to a share in their copyright.
“This creates a ticking timebomb ready to explode whenever the musician chooses and when, possibly, material witnesses have passed away.”
Mr Brooker also faces paying about £500,000 in legal costs. He added: “It’s hard to believe that I’ve worked with somebody on and off since 1967 whilst they hid such unspoken resentment.”
He said that the real credit should have gone to Johann Sebastian Bach, whose Air on a G String, which he heard on an advertisement for Hamlet cigars, and Sleepers Awake, inspired A Whiter Shade of Pale.
Mr Fisher said that he was securing his rightful place in rock ’n’ roll history. “It’s a great pity that this matter could not have been resolved amicably,” he added.
Mr Brooker has been granted leave to appeal. It is the second time in 18 months that the band’s bank balance has felt the pinch from claims by former members. Mr Brooker reached an out-of-court settlement with Ray Royer, a former guitarist and Jonathan Weston, a former manager, who claimed royalties for their contribution to A Whiter Shade of Pale.
More about the AWSoP lawsuit