Transport for London admitted the project to build a new line between Canning Town and Stratford International has suffered problems during the past year.
The extension includes existing stations at Canning Town, West Ham and Stratford as well as new stations at Stratford International, Stratford High Street Abbey Road, Star Lane.
A TfL spokesperson said: “The Stratford International extension will open to the public in late Spring 2011. This delay has been caused by joint venture contractors Skanska and Volker Rail taking longer than expected to progress some technical aspects of the project such as communications system design and installation.
“While further work on these systems is required, test trains have started running on the extension. The new link will be ready well in advance of the 2012 Games and provide a lasting legacy to the East London community.”
Stratford International opened last year but is not connected either to any rail line or any international station - instead a bus service exists to and from Stratford, with passengers only able to go as far as Kent without changing.