TV viewers in around 18 million homes with Freeview will need to retune their set-top boxes and TVs from today.
The multi-channel service is being upgraded to give more homes Channel Five, and also pave the way for the supply of high-definition (HD) television.
TV sets, set-top boxes and digital recorders will need retuning from lunchtime.
More than 500,000 Freeview homes will be able to receive Five for the first time, but about 460,000 households are also expected to lose access to ITV3 and ITV4.
It is also thought that about 20,000 older set-top boxes will no longer work.
Ilse Howling, managing director of Freeview said: "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."
The boxes which will no longer work are: the Daewoo models DS608P and SV900, Labgear DTT100, Triax DVB2000T, Portland DP100 and Bush IDVCR01.
Although the upgrade will affect around 18 million homes, it is thought that about 25 million televisions and set-top boxes will need retuning.
Viewers who are unsure how to retune their TVs or set-top boxes are being advised to consult instruction manuals or visit the website www.tvretune.co.uk for a list of retune guides.