They were spotted walking in hills in the east of the county by a police helicopter on Thursday.

Ninety firefighters were tackling the fire, which covered an area of four square miles.

The blaze began at about 1200 GMT and fire crews were battling into the night to put it out.

The area affected was between Tockholes, Abbey Village and Belmont, near Bolton.

'Water useless'

Lancashire Fire Service said the fire was moving towards Chorley, but no property was thought to be at risk.

Firefighters formed a line of about one mile and were trying to put the fire out by beating it with paddles.

A fire service spokesman said: "About a quarter of our full complement of fire crews are tackling the fire.

"The problem with moorland fires is that it is not possible to put out the fire with water.

"The heather is like a thatched roof, which means that the water just flows off it.

"No property is at risk, but the real worry is environmental damage."

About 70% of the world's heather moorland in the world is in the UK, said the spokesman.

Though grass and heather will grow again after a fire, the peat beneath can be lost forever, damaging the eco-system.