A three-tier warning system designed to help people avoid getting heatstroke will be launched on May 15, along with new guidelines for those working outside.

The Labour Department's "Heat Stress at Work" warning will come with amber, red or black alerts, advising of outdoor risks to health for at least an hour.

Employers will be advised to take necessary measures to prevent workers from getting heatstroke if the warnings are triggered.

Under a red or black alert, for example, bar benders and porters will be advised to stop work completely.

Deputy Commissioner for Labour Vincent Fung said he believes the red and black warnings will be relatively rare.

"According to data in the past six to seven years, no black warning would have been triggered, and there wouldn't have been much chances for a red warning either. Most of the time it would be an amber warning and mostly during midday," he said.

Fung suggested firms arrange for their staff to take lunch breaks at the hottest time of the day.

He added that although the guidelines will not be mandatory, he hopes employers will follow them.