The Communications Commission of Kenya has defined fake handsets as "copies of popular brands and models made from sub-standard materials" that have not been licensed by the organization, the BBC reported.
The action is intended to protect consumers from hazardous materials and to safeguard mobile payment systems, Kenyan officials said.
Phone users were being urged to check their phones' IMEI (International Mobile Equipment Identity) codes to confirm they are recognized as being valid.
They can send a free text message containing their code to check that their handset is recognized as genuine, officials said.
The "switch-off" had been scheduled for the end of 2011 but has been twice delayed to give phone users time to replace their devices.
Of the 29 million cellphone subscribers recorded at the end of March, 3 million were using counterfeit handsets, the government said.
The fake phones have been popular since they are often sold at prices well below legitimate models, in part because retailers can avoid paying import taxes.