With a potential speed of four billion floating point operations per second, the new G4 is claimed to have at least twice the performance of its rival.

Software companies were positive about the G4 but analysts said the development was an expected improvement in the technology.

Security restrictions

Apple's chief executive Steve Jobs claimed the new chip, jointly developed with IBM and Motorola, is so powerful that the US government had classified it as a supercomputer and restricted exports to certain "sensitive" countries.

During the launch in San Francisco, a head-to-head race against a Pentium III system running at 600 MHz was claimed to show the G4's superior performance. The task was rendering a scene with Buzz Lightyear from "Toy Story".

The first reaction from software developers was enthusiastic.

"The new G4 is an unbelievable machine for creating compelling content that adds life to the Web," said Rob Burgess, Chief Executive of Macromedia.

Adobe Systems boss, John Warnock, said that the G4 runs his company's image editing software Photoshop significantly faster than any other platform.

Critics: G4 as expected

iBook: Apple's summer break with convention

The stock market warmed to the new product, boosting Apple shares by five percent on the Nasdaq stock exchange. The G4 launch comes just weeks after the successful launch of Apple's colourful iBook notebook computer.

But some industry watchers were more cautious.

CIBC Oppenheimer analyst James Poyner said the G4 launch had been expected, and that its improvements over the Apple G3 were "basically following processor speed upgrades."

"It's not the kind of change that is going to lead customers to say, 'Gee, I wasn't going to buy this but now I am,'" he said.

The 400 MHz Power Mac G4 costs $1,599 in the US, and higher-priced 450 and 500 MHz versions are expected to ship in September and October. In the UK, the 400 MHz G4 will cost £1,099, available from mid-September. The 500 MHz G4 will cost £2,399 from late October.

But Intel will not let the G4 go unchallenged. "Coppermine", an enhanced version of the Pentium III processor, will be released in October.