The long-awaited DLR extension to Stratford International has been hit by further delays and will now not open until the summer, transport chiefs have admitted.

Transport for London promised earlier this year that the line — which will link the high-speed station with Canning Town — would open in “late spring”.

Services were originally slated to start last year, but bosses say contractors have been hit by technical problems, meaning the opening will now be this summer.

London Assembly member John Biggs challenged Mayor Boris Johnson to take responsibility for the hold-ups.

Speaking at mayor’s question time, the Labour representative for City and East asked: “Given that you boasted and sought credit for the early opening of the Overground connection between Dalston and Highbury & Islington, are you happy to accept the blame for the delay to the extension of the DLR to Stratford International?”

In a written answer, Mr Johnson said: “The Stratford International extension will open in summer 2011.

“There has been a delay in delivery and this has been caused by joint venture contractors Skanska and Volker Rail taking longer than expected to progress some technical aspects of the project.

“While further work has been required, test trains have been running successfully on the line and this phase of the work will be complete shortly.

“The extension will open to the public a year ahead of the 2012 Games and provide a lasting legacy to east London.”

Construction of the line has included building new stations at Star Lane and Abbey Road and refurbishing the old station at Stratford High Street.

Services will also call at Canning Town, West Ham and Stratford regional station, running on trackbed previously used by a closed section of the North London Line to North Woolwich.

Eurostar blamed poor transport connections to Stratford International as a reason for its services not calling there.

The high-speed stop is around 400 yards away from the regional station, with a shuttle bus running between the two.

320 words