The alert system comprises three levels: "Medium", "High" and "Very High".
Speaking at London's City Hall, Mayor Sadiq Khan said there was "simply no other option", the BBC reported.
"Nobody wants to see more restrictions but this is deemed to be necessary in order to protect Londoners," Khan said, adding: "We've got a difficult winter ahead."
Besides London, Essex is also expected to move to high alert restrictions this weekend.
Meanwhile, Greater Manchester and Lancashire could join Liverpool City Region and be placed under Tier 3 - "very high".
Health Secretary Matt Hancock is due to update MPs on the latest measures in a House of Commons statement later in the day.