Plans to rework the subsurface network* have been being discussed for years now, but with no real indication of exactly what is planned and when it might happen.
This posting quotes TfL's current thinking and it looks fairly definite to me. The plan is to have Circle Line trains start at Hammersmith, run along the Hammersmith & City Line to Edgware Road then do a clockwise lap of the Circle, arriving back at Edgware Road. They'd then reverse and go anticlockwise round the circle back to Hammersmith. Despite my headline, the service would continue to be called the "Circle Line".
The big advantage for passengers is the doubling in service to Hammersmith as the Hammersmith & City Line will continue to run - currently the branch only gets 7 trains per hour. The big drawback is no through service between the west and north sides of the Circle - you will always have to change at Edgware Road.
Reader John Hawkins emailed an analysis of the numbers in this press release about the new S Stock trains that seems to confirm the plan:
There will be 133 7-car trains. If this is all about the Hammersmith-Barking service, and D-stock replacements remains 78 S7 instead of 75 D6, then leaves 55 S7 to replace 46 C6, which must be the T-cup service enhancement to Hammersmith.The other rumoured change, to have the Metropolitan run to Barking and the Hammersmith & City to Aldgate appears to be dead.
* The Circle, District, Hammersmith & City and Metropolitan Lines, which all share track