Radiohead is to release 18 hours of secret recordings from the time it released OK Computer after a "hack".
Radiohead was "hacked" and cyber attackers stole a minidisk archive from around the time the album was recorded, Greenwood said. The hackers threatened to release the files unless the band paid £150,000, he claimed.
The "only tangentially interesting" music was never intended for public consumption, he added.
In a statement on Twitter, Greenwood said: "We got hacked last week - someone stole Thom's minidisk archive from around the time of OK Computer, and reportedly demanded 150,000 on threat of releasing it.
"So instead of complaining - much - or ignoring it, we're releasing all 18 hours on Bandcamp in aid of Extinction Rebellion.
"Just for the next 18 days. So for £18 you can find out if we should have paid that ransom.
"Never intended for public consumption (though some clips did reach the cassette in the OK Computer reissue) it's only tangentially interesting. And very, very long. Not a phone download."
He added: "Rainy out, isn't it though?"
Greenwood also tweeted links to Radiohead's page on music sharing site Bandcamp and to the website of climate campaign group Extinction Rebellion.
A note from Yorke on the Bandcamp page says: "We've been hacked.
"My archived mini discs from 1995-1998(?). It's not v interesting, there's a lot of it.
"If you want it, you can buy the whole lot here. 18 minidisks for £18. The proceeds will go to Extinction Rebellion.
"As it's out there, it may as well be out there. Until we all get bored and move on."
Additional reporting by agencies
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