The route will still be called the Circle Line
The London Underground's Circle Line is to be extended.
From December, the line will run to Hammersmith in west London using tracks it shares with District, Hammersmith & City and Metropolitan lines.
London Underground (LU) said giving the Circle Line a fixed terminus would also help improve reliability and capacity.
It is the first phase of the Sub Surface lines upgrade, which will include a fleet of air conditioned trains and a new signalling system.
The new service will run from Hammersmith to Edgware Road station where it will join the Circle Line and make a single loop back to Edgware Road.
It will then reverse and do the full circle anti-clockwise, returning to Hammersmith.
Howard Collins, of LU, said: "The Circle Line shares tracks with the District, Hammersmith & City and Metropolitan lines and if there are service problems the Circle Line inevitably becomes disrupted.
"The changes to service patterns on the Circle Line will improve the reliability of the line, enabling services to recover more quickly if there is a disruption.
"It will still be called the Circle Line and shown as a circle on the Tube map but with the additional extended service out to Hammersmith.
"The added benefit will be the improved service on the Hammersmith & City Line from Edgware Road round to Hammersmith, almost doubling the frequency on that route."