This video shows the dramatic moment when clouds of thick black smoke billowed out from the Bildsdale transmitter.
More than 30 firefighters are tackling the blaze at the TV mast on the North York Moors.
It has caused TV channels to be blacked out across North Yorkshire.
Ron Needham, 71, had just had lunch at the foot of the mast with his wife Sue when he saw the smoke, and took these videos and pictures.
He told YorkMix: “It’s a walk we do fairly regularly. We walked up and sat and had our lunch at the mast, and then continued down the ridge, coming down the other side to the valley.
“Just as we got round to the other side of the valley, we glanced across and saw smoke coming out of the top of the mast.
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“It was quite strange.
“Then suddenly a huge cloud of thick smoke came from the bottom. Clearly it was on fire.
“I have no idea what caused it at all.”
North Yorkshire Fire and Rescue issued an update on Tuesday evening.
North Yorkshire Fire and Rescue Service attended a fire at the Bilsdale transmitting centre this afternoon, 10 August 2021.
The initial call came at 13:19 from an engineer working at the transmitter, stating that he believed the mast was on fire due to smoke coming from below the first stay level (approximately 50 – 60 metres up). Calls were also received from members of the public who could see the smoke from some distance.
North Yorkshire Fire and Rescue Service worked to control the fire in difficult circumstances and extinguished a fire in a single-storey stone building and the 315m (1032ft) transmitter mast. Only one building in a complex of four was affected and there are concerns about the structural integrity of the mast.
A 300m exclusion zone has been put in place around the mast.
Eight pumps from North Yorkshire Fire and Rescue Service attended the incident and firefighting has now ceased until the site has been confirmed as safe for further work.
The incident was attended by crews from Helmsley, Malton, Thirsk, Danby, Coulby Newham, Ripon, Northallerton and Acomb in addition to mobile water bowsers from Tadcaster and Boroughbridge.
NYFRS will be working on site tomorrow with Arqiva, the site operators, to bring the incident to safe conclusion.