The city is expected to follow the lead of towns and cities around the country who already have 20mph zones.
Each of the capital's 33 councils could launch their own borough-wide "safety zones", after evidence that they cut accidents and injuries by half.
Until now boroughs have only been allowed to introduce 20mph limits in limited pockets - and only if they install costly enforcement measures including humps and speed cameras.
But the councils will be able to make 20mph the "default" speed across their borough - with or without enforcement measures.
TfL has approved trials of wireless cameras capable of enforcing area-wide speed limits. One in 10 roads - roughly 12 per cent of the road network in the capital - is already covered by 20mph zones.
Statistics claim that nine out of 10 pedestrians will be killed if hit by a car travelling at 40mph, two out of 10 will die if struck at 30mph but his drops to one in 40 at 20mph.
The AA said a blanket 20mph speed limit was wrong.
Paul Watters, head of roads and transportation policy, said 20mph zones had "huge" benefits - but only when applied judiciously.
He said: "If this becomes the default limit across the board it will reduce capacity on London's roads.
"London is geographically big and still has lots of main A-roads where people have to make progress. If some main roads end up with 20mph limits, people will ignore it and this could bring other 20mph zones into disrepute. They should only be introduced on a road-by-road basis."