The blaze, which has been burning since Sunday, has covered nearly six sq miles and is the largest in recent years on a patch that is important to wildlife but sensitive to combustion.
The National Trust, which owns the moorland, said its recent investment in restoring the area had been wiped out. It now hopes the rain forecast for later today will help to finally extinguish the flames.
Thelma Walker, whose Colne Valley constituency covers the moor, said she was “devastated” to see the destruction.
“It is heartbreaking to see precious moorland habitat destroyed and to think about the impact this will have on wildlife during the breeding season for the ground nesting bird population,” she said.
It is believed the blaze was started by people lighting a portable barbecue. A group who raised the alarm were “genuinely mortified” at what had happened, the Trust said.
Ms Walker, who has represented the area for Labour since the last election, said she would raise a question in the Commons on tightening the regulations on disposable barbecues.
The blaze is already the sixth this year on Marsden Moor. An earlier incident, during the warm spell in February, was said to “amongst the worst” of recent times, but the Trust said the current blaze was far worse.
“It’s the most significant fire in recent years on Marsden Moor. From where it started on Sunday at Easter Gate, it spread across Close Moss and has since headed west towards Castle Shaw,” a spokesman said. “On Monday evening the fire jumped across the A640 towards Readycon Dean Reservoir, but crews fought this breakout back to the road.”
Ten fire crews from West Yorkshire, Greater Manchester, Cumbria and Merseyside are on site, and a helicopter and a specialist high volume pump has been taking water from nearby reservoirs to the flames. The aircraft costs the Trust £2,000 an hour.
Marsden Moor, with its ground nesting bird population and blanket bog habitat, is designated as a site of special scientific interest, a special protection area and a special area of conservation.
It is home to merlin and other upland birds, but officials say the biggest loss of wildlife is likely to be among be nesting birds such as curlew, and the mountain hares that inhabit the moorland.
The Trust said: “We’re devastated to see the destruction caused. We need our visitors’ help to prevent the risk of fire across the countryside that we care for, particularly when we experience prolonged periods of dry weather or are in drought conditions.
“People can make all the difference by making sure any lit cigarettes are properly extinguished and disposed of responsibly, to never light fires and only use barbecues in authorised areas.”
The Marsden outbreak came a few hours after a blaze spread across 60 hectares on Ilkley Moor. Above Blaenau Ffestiniog in North Wales, fire crews have been battling a mountain blaze that led to the evacuation of around 20 homes.
There have also been two separate moorland blazes in Scotland since the warm spell began.