Apple claims that it has "redefined" mobile computing with its new Titanium PowerBook G4. The new Mac notebooks run G4 processors at 400MHz and 500MHz, are just an inch thick, boast "mega-wide" 15.2-inch screens, and are made – as their name points out – from "space-age" titanium. Prices start at £1,899.

"It is a ‘supercomputer to go’ in terms of performance, yet it’s thinner and sexier than the best subnotebooks," said Apple CEO Steve Jobs during his keynote address to this week’s Macworld Expo in San Francisco. Jobs called the new PowerBook "unbelievably beautiful".

Titanium is "stronger than steel" yet "lighter than aluminium" according to Apple. Despite the larger TFT widescreen active-matrix display (up from the old G3 PowerBook’s 14 inches), the Titanium PowerBook G4 weighs 5.3 pounds compared to its predecessor’s near-7lb bulk.

"The all new Titanium PowerBook G4 is the most revolutionary portable computer ever created," said Jobs. "It has been redesigned from the ground up," he added.

Apple claims that the Titanium PowerBook G4 outperforms Pentium III-based notebook systems by up to 30 per cent based on 12 commonly used actions in Adobe Photoshop 5.5. Apple also claims that its G4 processor means that the new laptop is 60 per cent faster when working with Final Cut Pro than the old G3 models.

Its key new features include PowerPC G4 processors with Velocity Engine running up to 500 MHz with a 1MB L2 cache, slot-loading DVD-ROM drive, and AGP 2X Rage Mobility 128 with 8MB SDRAM. Apple claims that its Lithium-ion battery will keep running for up to five hours.

The G4 PowerBook’s display is 52 per cent larger than that of the Sony Vaio subnotebook that Apple is clearly targeting as its closest rival. It is also 15 per cent slimmer than the 650MHz Pentium III Windows PC acclaimed for its skinniness.

Apple’s British designer called the new laptop "uncompromised": "People will have a visceral reaction to its weight and volume".

The 400MHz model ships with 128MB of RAM; the top-end 500Mhz model with 256MB. Both are expandable to 1GB. The entry-level PowerBook has a 10GB hard drive; the higher-specified model has a 20GB disk. Build-to-order options on online Apple Store will include a 30GB hard drive (an extra £350 on the 10GB hard drive, and £213 on the 20GB drive).

When and how much? The new G4 PowerBooks will be available by the end of January, and will ship with iMovie 2 and iTunes as well as Mac OS 9.1.

The 400MHz G4 Titanium PowerBook costs £1,899.; the 500MHz model costs £2,499. Both prices exclude VAT.

All models ship with Mac OS 9.1, which was also launched at Macworld Expo in San Francisco.