The next version of Apple's Mac OS X - dubbed 'Leopard' – could earn the company $140m by the end of the year, according to an executive at the company.

Leopard – which is priced at £85 for a single-user licence and £129 for a five-licence family pack - is due for release on Friday, and Apple says the vast majority of Macs sold over the past four years will be able to run the highly-anticipated operating system.

"Specifically, the number is about 21 million," said Apple's chief operating officer Tim Cook. "When we announced Tiger, there were 15 million units that were eligible to run Tiger and we did $100m of revenue on Tiger at the first quarter of launch."

If Leopard sells to current Mac owners at the same rate, Apple will collect about $140m in revenues until 31 December.

However, analysts are sceptical. Ezra Gottheil of Technology Business Research, for example, told Computerworld US Leopard's contribution to the quarter would be $157m."

Like others, Gottheil came up with his projection by looking at the current installed base of eligible machines and comparing it to what existed when Mac OS X 10.4, also known as Tiger, debuted in April 2005.

"It really is a function of the installed base," said Chris Swenson, an analyst with the NPD Group. "Each upgrade sold better than the previous one, which is pretty impressive. The first two months after Tiger was launched, it ran 30 percent higher [in sales volume] than [Mac OS X] 10.3, and more than twice as high as 10.2."

Early sales of Leopard are promising, according to MacObserver. The website claimed earlier this week that Leopard is being pre-ordered at twice the rate Mac OS X 10.4 was in the week before its release. The site said the current anti-Windows Vista movement was among the factors encouraging people to sign up to the new Mac OS.

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With additional reporting from Gregg Keizer at Computerworld US.