More than 18 million households with Freeview will need to retune their set-top boxes and TVs later.
The service is being upgraded to give more homes channel Five, while those that did get Five will find themselves unable to watch it until they retune.
TV sets, set-top boxes and digital recorders will need retuning from lunchtime. It is thought about 20,000 older set-top boxes will cease to work.
About 460,000 households are expected to lose access to ITV3 and ITV4.
The technical changes behind the upgrade will eventually enable high-definition (HD) television to be broadcast on Freeview. HD is already available on the free-to-air satellite service Freesat.
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