The project is now expected to be completed before the London 2012 Games.
TfL has praised the engineers whose rapid progress now means those works may be able to finish six months earlier than scheduled.
When work started in February, it was expected to take three years to complete the £70m refurbishment of the 113-year-old tunnel.
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However, TfL and its contractor, BAM Nuttall, have made excellent progress on the refurbishment which means the safety upgrade will be finished in time for the London 2012 Olympic and Paralympic Games in July 2012.
Work completed so far includes the installation of new ventilation fans and concrete floors in two of the tunnel's four shafts, and the installation of temporary lighting, CCTV and communications systems throughout the tunnel, which will allow the old systems to be removed.
Work to install the permanent systems, which will bring the tunnel up to current standards, will begin later this year.
(CD/KMcA)
