An engineering design blunder could mean part-closures on the Gospel Oak to Barking Overground line stretch into early 2018 – with seething funders Transport for London (TfL) promising to seek compensation.

The route will reopen on February 27 after being down since June as part of plans to electrify the route and replace its two-car diesel trains with modern four-car electric ones.

Network Rail said structures that carry overhead wires necessary to upgrade the 14-mile line were “incorrectly designed” and could not be installed. Materials were also delivered late, which amplified the delay.

TfL, which put up the cash for the works, said it was “extremely disappointed” and will be “seeking compensation”.

Jonathan Fox, TfL’s director of London rail, added that Network Rail did not inform TfL of the “significant problems” until “very recently”.

He said: “Londoners can be assured that we are pressing Network Rail for an urgent plan that explains exactly how this vital project will be delivered on time and with the absolute minimum further disruption to our customers.”

Network Rail said a “robust” scheme is now being drawn up to finish the work before the new electric trains arrive in early 2018 – meaning further closures over a series of weekends and likely another full closure later in the year.

Richard Schofield, Network Rail’s route managing director for Anglia, said a “full review” into what happened had already begun.

He added: “I sincerely apologise to passengers that we have not been able to complete all of the work in the time we set out, and for the future disruption we will cause to their journeys.

“I have instructed the project team to quickly deliver a robust plan to finish the work before new trains arrive next year, and passengers can be reassured that the line will reopen later this month to diesel trains as planned.”

Mr Schofield also apologised to people who live near the line for the “inconvenience” set to be caused by extended engineering works.

A full schedule is being finalised and will be released as soon as possible.

The upgrade is expected to double capacity and improve air quality along the line.

Network Rail said the work was being undertaken in a bid to “transform the way people travel” across east and north London by improving its “Victorian railway”.

So far, five sections of the line have been lowered between Walthamstow Queens Road and Gospel Oak stations in preparation for the installation of the wires.

Ten bridges have also been reconstructed and strenghthened along the routem while three electrical switching stations have been built.

Before Network Rail made the announcement, Barking–Gospel Oak Rail User Group (BGORUG) said Network Rail was “well behind” in its upgrade.

Members of the group had taken a series of photographs along the line showing unfinished works.

BGORUG explained that the work still outstanding includes:

– Platform extensions at Gospel Oak, Barking and Blackhorse Road

– The installation of overhead wires through Dartmouth Park and at Gospel Oak station

– Step-free access installation at Blackhorse Road (expected to be completed in April 2017)

– Erection of overhead wires support masts

– Fitting out each mast with support arms for electric wires

– Installation of 25,000volts AC contact and support wires

– Repairs to a damaged sewer in Walthamstow

– Raising Crouch Hill road bridge

Its secretary, Glenn Wallis, said: “With rail replacement buses that missed out half the stations and limited refund arrangements, passengers have been left to largely fend for themselves.

“Now we learn that although trains are to start running again, the passengers’ pain is far from over.”

Network Rail expects the new four-car electric trains to be running from late spring 2018, with electric freight trains running from winter 2017–18.