Transport for London (TfL) has promised to provide an update “later this year” on proposals for a new underground ring road around London to relieve congestion.

A spokesman for TfL said it was still carrying out detailed work to assess the viability of the plan. He was unable to clarify the exact nature of the work, however, or say when a final decision might be made.

Plans for the conceptual inner orbital tunnel were first revealed in Spring last year after the Mayor’s independent Roads Task Force recommended looking at road tunnelling projects in parts of the capital. The tunnel would provide a 22 mile route under the capital’s streets, freeing up surface capacity, improving air quality and reclaiming space for public parks, pedestrian and cyclists, TfL said at the time.

TfL originally suggested its initial work to assess the options and affordability of the scheme would be completed by the end of last year but it has avoided issuing any update or commenting in detail on the progress of the project ever since.

A Roads Task Force Update dating from April last year suggested the tunnel route, which TfL is considering making tolled, would result in a 10% drop in congestion compared to 2007 levels.