Apple has warned MacBook owners to check for stray pieces of plastic left behind during manufacturing if their new Macs run too warm.

A vocal minority of new MacBook and MacBook Pro owners have complained that their machines run at very high temperatures, prompting the company to investigate these matters.

First fruit of these investigations: the company published a technical note yesterday, in which it warned that some MacBooks "may have left the factory with a thin piece of clear plastic covering the rear vent".

According to Apple, this thin plastic slice is used in the factory to prevent dust from getting into the machine. "If your MacBook has the plastic still over the vent, simply remove and discard it," advises Apple.

The company has also published a second document for notebook owners who think their machines are running too hot, in which it explains that a certain amount of heat is perfectly normal.

"The bottom surface and some areas between the keyboard and LCD hinge of your Apple notebook computer can become hot after extended periods of use," Apple explains, calling this "normal operating behaviour.

"With processor and bus speeds in portable computers often matching, if not exceeding, those of desktop systems, increased operating temperatures in portable computing products are common throughout the industry," the company says.

Apple advises users place their portable Macs on a flat stable surface, but not on their laps or upholstery.

"When using your portable computer, place it on a hard surface, such as a desktop or a tray. A hard surface allows air to flow under the computer to dissipate heat. Placing it on a soft surface such as a towel or pillow (or lap) is not recommended because it does not allow air to flow under the computer," the company warns.

Apple also recommends users set their Energy Saver settings to 'Better Energy'.