Intel is to update its Core Duo processor line, the chip maker has told customers, with samples of new T2400, T2500 and T2600 65nm dual-core processors shipping by early May.

According to company documentation seen by Reg Hardware, the move will see Intel replace the three processors' C-0 core stepping with an updated version, dubbed D-0. The scheme will apply to shipments of commercial products commencing 23 June. At this stage, it's unclear what the core update involves beyond the broad "speed enhancements" Intel mentions in its documentation.

D-0 Core Duos are backwards compatible with C-0 parts, though the new core will require a BIOS update, Intel warned. The chip maker also said it will update Core Solo and Celeron M 400 series processors in due course - even though it has yet to ship a Celeron M 4xx CPU.

As it has been revealed in January, Intel is preparing three initial Celeron M 4xx parts: the 410, 420 and 430, clocked at 1.46GHz, 1.60GHz and 1.73GHz, respectively. All three are produced at 65nm, based on the 'Yonah' core. There's also a Low-Voltage 65nm Celeron M, the 423, in the works. It's clocked at 1.06GHz. All four chips use a 533MHz FSB and they're due to ship in the current quarter.