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
London City Airport is closed after reports of a "chemical incident", London Fire Brigade said.
Council leaders today called on the government to provide extra money to help resettle the youngsters in the capital.
An eyewitness said: "It was very unnerving. It was just after 12pm and there were suddenly police vans and cars everywhere."