North Yorkshire Fire and Rescue Services confirmed that crews from Helmsley, Thirsk, Danby and Coulby Newham had all been sent to Bilsdale mast near Helmsley at around 2pm on Tuesday after smoke was seen issuing from it. The structure is alight at the base.
A statement from NYFRS read: "We ask that members of the public do not go near to the site, particularly as this area has limited access and we need to be able to move vehicles and people around to help fight the fire.
"Crews are working hard to control the fire in difficult circumstances - it is unclear at this time if any of the centre's transmitting capability has been affected or the cause of the fire which will be investigated in due course.
"We currently have six fire appliances and our mobile water bowser as well as command and control assets in attendance."
The mast provides TV signal for a whole swathe of North Yorkshire and north-east England.
Although the fire service were not aware of the outages, the BBC confirmed that Freeview had been affected in a statement: "We have been notified of a fire at the Bilsdale transmitter that provides television and radio services in North Yorkshire and Teesside. All radio and television services are currently unavailable. Emergency services are on site and dealing with the incident. This is also affecting the smaller transmitters which receive signals from Bilsdale.
"Once emergency services have dealt with the incident, our engineers will assess the damage and work towards restoring the services as soon as possible.
"Please use the transmitter checker tool to find out if this problem is affecting reception at your location or if you are able to receive a radio service from another transmitter. Sky, Freesat and Cable services are not affected. If you want to continue watching or listening try BBC Sounds for radio or BBC iPlayer for television."
The website Downdetector reported outages in areas such as Harrogate, York, Sunderland, Middlesbrough and Stockton-on-Tees.
The 314-metre structure is among the most powerful transmitters in the UK and was built in 1969 to bring colour television to Teesside for the first time.
The coverage area encompasses Harrogate, York, Filey, Whitby, Middlesbrough, Stockton, Darlington, Bishop Auckland, Hartlepool and Barnard Castle.