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.
"These are significant and necessary changes which will immediately bring Freeview viewers new channels, introduce new homes to existing channels and prepare the platform for the future availability of Freeview HD," said Ilse Howling, managing director of Freeview.
All Freeview TV sets, set-top boxes and digital recorders, including TopUp TV and BT Vision boxes, will need to be retuned from lunchtime on 30 September.
The upgrade will affect around 18 million homes, and it thought that about 25 million televisions and set-top boxes will need retuning.
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