The urban speed limit will be cut from 30mph to 20mph under plans announced by London Mayor Ken Livingstone.
Livingstone will encourage London boroughs to make 20mph the “default” limit in all residential areas, the mayor’s office announced.
The move is likely to embolden transport authorities considering a 20mph limit in other parts of the country.
The mayor’s office said casualties had fallen in existing 20mph zones and that there expansion would help reduce greenhouse gasses.
Livingstone said the new lower limits would “build on London’s success as the only major city in the world to have achieved a shift away from car use to public transport, cycling and walking”.
He said: “This trend will be supported by the encouragement of 20mph speed limits in all residential areas.”
Exemptions for main routes will be negotiated between TfL and individual boroughs.
The mayor’s office claimed: “Nine out of ten pedestrians will be killed if hit by a car travelling at 40mph, two out of ten at 30mph and one in forty at 20mph.”