Digital UK has warned more than a million TV sets risk going blank when the London region begins the switchover in a week's time.
More than one in ten households still have a television relying on the old signal, amounting to more than a million TVs in the region.
The digital switchover replaces the existing analogue TV signal with a digital signal.
The first analogue channel to go is BBC2 on Wed 4th April; the remaining four, including ITV1, disappear on Wed 18th April.
Deborah Bain, London Manager for Digital UK, told ITV News: "If you're watching one of those [analogue] televisions, you need to do something or you will be left with a blank screen.
"You can convert your television with a digital box, satellite or cable services.
"It is really important if you already have Freeview you do a full retune on the 4th of April and then again on the 18th of April."
More top news
Two "ruthless, calculated and violent" men found guilty of killing a woman, 26, with sawn-off shotgun blast to face at 'point-blank range'.
A room that has been likened to "Harry Potter’s cupboard" under the stairs is up for rental in Seven Sisters for £350 a month.
Detectives have launched a hate crime investigation after a man threw a sausage at Finsbury Park Mosque.