Hundreds of thousands of people could struggle to watch Freeview television programmes next week when tests begin to assess the impact of new 4G mobile phone networks.
Around 170,000 households and businesses in south-east London have been warned their digital TV signal could be disrupted on Monday because of the trial.
They could lose some or all Freeview channels and experience loss of sound or blurred images.
People in parts of Greenwich, Lewisham, Southwark and Tower Hamlets are being alerted to the test.
Hospitals, care homes, schools and businesses have been issued with advice on how to avoid any problems, while further arrangements are being made to support vulnerable people during the trial.
The test follows a smaller pilot in the West Midlands where people living in 22,000 homes were warned they could experience interference last month.
On that occasion, only 15 households reported problems with television reception caused by 4G, as opposed to the predicted 120.
The trials are being conducted by at800, a firm tasked with ensuring viewers continue to receive free-to-air television when 4G broadband is switched on at 800 MHz.
Simon Beresford-Wylie, chief executive of at800, said: "These larger tests are essential to help improve our forecast model and the way we'll tackle potential issues caused by 4G at 800 MHz.
"We are extremely grateful to viewers in south-east London for their help with these important tests."
Viewers in the area have been asked to report problems with television reception from Monday.
Teams will be placed on stand-by to respond to calls and restore Freeview services if they break down.
Viewing issues could arise because the high-speed mobile phone service at 800 MHz and digital television operate in adjacent parts of the radio spectrum.
Equipment that receives television signals - such as aerials, amplifiers, and digital tuners - can fail to block 4G at this frequency.
Satellite and cable TV services are unlikely to be affected by 4G at 800 MHz.
Anyone in the south-east London area with Freeview disruption from Monday is asked to call at800 on 0333 31 31 800.
Those at risk of interference from 4G services will be contacted directly by post.