Labour officials on Monday said they were updating guidelines on when outdoor workers would be advised to stop working when it's very hot.

This would enable the department’s heat stress warning system to be in line with the observatory's warnings about hot weather.

The observatory’s "extremely hot special alert" is issued when it's generally 35 degree Celcius and above.

Some environmental and workers' groups had called for such a linkage between the two alert systems last year when the department’s guidelines were introduced.

The Labour Department said the updated guidelines would apply next month.

The guidelines also set out the varying lengths of breaks workers should be given every hour when the heat stress warnings are in effect.

The new rules state that if employers adopt a certain amount of measures to prevent heatstroke - such as reducing workload, providing shades and using mist fans to improve ventilation - they can completely cut the hourly breaks for workers.

If insufficient measures were adopted, the guidelines said, employers would be required to give longer breaks to workers.

Employers would be given flexibility to alternate break times for different workers, and they can split break periods into shorter ones so as to minimise the effects on work progress.