About 400,000 homes have been left without television reception after an "intense fire" at the Beckley transmitter in Oxfordshire.
Firefighters said part of the structure was in danger of collapse following the blaze, which started at about 1400 BST.
Cable and satellite services and Five’s analogue signal are not affected.
Engineers said it was unsafe to carry out repairs and signals would not be restored until at least 1900 BST. The fire has now been put out.
Eyewitnesses saw smoke drifting from the top of the mast at about 1420 BST – when pictures went off – and reported parts of the structure falling off.
Digital switchover
A cordon has been put in place around the site.
Matt Carlisle, of Oxfordshire Fire and Rescue Service, said: "It has been subject to quite an intense fire at the top, so there is a possibility that the structure may be weakened."
He said the cause of the fire was not yet known.
The Beckley mast transmits across Oxfordshire and parts of Wiltshire, Buckinghamshire and Northamptonshire.
Maintenance has been taking place on the mast in preparation for digital switchover in 2011.
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