An email published on a blog knocked $4bn off Apple's stock market valuation on Friday before it was exposed as a hoax.

Technology news blog Engadget published an apparent internal email discussing how the forthcoming launches of Apple's iPhone and Leopard OS update would be delayed until October 2007 and January 2008 respectively.

The news spread around the web and Apple's stock lost four per cent of its value in six minutes of trading.

But the email, which was sent to a number of Apple employees, one of whom forwarded it to Engadget, was soon exposed as a fake.

"The person or persons behind the phoney email had apparently put one over on Apple employees to the extent that those employees who received that memo and passed it along to us and others took it as truth - as did we," said Engadget editor Ryan Block.

"Although we made sure to confirm and reconfirm with our source that this email was legit at the time it was sent out, unfortunately no amount of vetting and confirming sources can account for what happens when a corporate memo turns out to be fraudulently produced and distributed in this way.

"Credibility and trust is the currency of our realm, and it's clear we lost some of that."

The incident has highlighted the jittery nature of Apple's stock valuation, which is currently nearly three times higher than its previously highest value. The stock has risen fourfold in the past four years.