A tree that has stood in Ashridge Park for more than 200 years has gone up in flames after someone dumped hot barbecue coals at its base.
Although dead, the oak tree provided an important habitat for countless invertebrates on the National Trust land.
Firefighters had to be called in on Wednesday morning after reports that the tree was ablaze.
Lead ranger Lawrence Trowbridge said: “Someone very close by had obviously had a barbecue and I think when they had finished they had dropped the hot coals of the barbecue at the base of the tree. It is a crazy thing to do.
“It slowly caught fire and got worse and worse over time.
“The fire brigade emptied a whole tank of water onto the tree and even then it wasn’t out.
“There is no doubt that lots of little beetles and larvae in there have been burnt alive, which is horrible.”
It is expected that the tree may still have to be felled to ensure that the fire is completely out.
The emergency has prompted the trust to issue a safety warning about summertime barbecues on its land.
Lawrence said: “If the grass had been very dry and long there could have been a horrendous fire.
“We only allow barbecues on the Monument Drive and we don’t allow barbecues anywhere else on the estate.
“It is lovely that people want to come and do that but to avoid a real fire risk come to Monument Drive and have a proper barbecue on a stand and always bring plenty of water or a fire extinguisher.”
Those using disposable barbecues are urged not to place them directly onto the floor as the base gets hot and to take home their rubbish at the end of the day.