Islington is to become the first borough in the UK to introduce a 20mph speed limit on all residential roads.
Islington council has agreed to introduce the limit in January. It says the cut will reduce accidents, congestion and pollution.
Transport for London research found that 20mph zones reduced casualties by 43 per cent and the number of fatal or serious casualties by 53 per cent.
More than 150 miles of road will be affected, with motorists able to drive at 30mph on only 15 out of 1420 streets in the borough. However, more than half of Islington's roads are already covered by 20mph limits.
Councillor Greg Foxsmith, Islington's environment spokesman, said, "We want streets where our kids can walk to school safely, where our cyclists can ride without risk, and where pollution levels are driven down.
"A blanket 20mph zone is a bold step, but it's what our residents want and deserve. The scheme has had cross-party support and widespread public backing."
The AA warned that a 20mph limit for London would hamper travel. It wants councils to introduce such zones only in sensitive or residential areas.
Signs will be put up announcing the limits, but the council rejected calls to enforce the limit with speed bumps.
Roads run by Transport for London such as the A1 and Camden Road will continue with a 30mph limit.
Other London boroughs are reported to be looking to follow suit. These include Southwark, Hackney, Brent, Kingston upon Thames, Lambeth, Tower Hamlets and Merton.