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
New research shows almost half of Londoners currently own their own business, or plan to do so.
David Cameron said the Met proved it can "do more with less" and declined to criticise the Mayor of London for his water cannon spend.
A group of disabled activists threw toys and balls at him as he entered the conference.