Media playback is unsupported on your device Media caption Aerial pictures show work on the new EastEnders set in Elstree, as filming takes place on the existing set.

The BBC's project to build a brand new, HD-ready Albert Square and expand the external EastEnders set will now cost £27m more than originally planned.

The original 2015 forecast for the scheme was £59.7m, but the revised budget is now £86.7m, the National Audit Office (NAO) has revealed.

The new set was originally due to open in 2018, but won't be ready until 2023.

The BBC said the project was "large" and "complex", adding that there had been "challenges on the way".

Image caption A brand new Queen Vic pub is being built as part of the new set

What is happening to the EastEnders set?

The corporation is building a new Albert Square near the existing set at the BBC's Elstree studios in Hertfordshire. It will have actual brick buildings - unlike the current one, which only uses facades.

The external EastEnders set dates back to the soap's birth in 1984 and was only supposed to be used for two years, but has lasted for 34.

The soap cannot currently be filmed in high definition, and the BBC said the poor condition of the current set is increasingly causing delays to filming.

The building project - titled E20 - was announced in 2015 and was meant to finish by August 2018.

Image caption The original set was only supposed to be used for two years

Why has it gone over budget?

In its report, the NAO, which scrutinises public spending for Parliament, said the extra costs mean "the BBC will not be able to deliver value for money... in the way that it envisaged in 2015".

The body took aim at the early planning stage, which "resulted in the BBC underestimating aspects of complexity, cost and risks of its approach".

However, the NAO also acknowledged that the BBC had faced issues such as asbestos and obstructions in the ground, which could not have been foreseen when the project was first planned.

Inflation has also risen faster than expected in the construction industry - although the NAO said inflation "had a greater impact than it would have done had the programme been completed without any delays".

Sir Amyas Morse, head of the NAO, said: "We believe that the planned benefits are still broadly achievable, but given the high risk nature of E20 it will need close scrutiny until it's finished."

The NAO said "some of the reasons for the delays and cost increases could have been addressed earlier by the BBC", adding that the corporation "had made many improvements to the programme" in the last 18 months.

Image caption The funeral of Peggy Mitchell was a big Walford event

What has the BBC said?

"The show remains one of the BBC's flagship programmes and yet is filming from a set that is no longer fit for purpose," the BBC said in a statement.

"The new set will be suitable for HD filming for the first time and extend Walford to better reflect modern East End London.

"It's a large, complex project which has already delivered many other vital improvements at BBC Elstree Centre but like any building work of this scale there have been challenges on the way, including construction market issues beyond our control and from working on a brownfield site.

"As the NAO recognises we've already made improvements and are keeping the project under close scrutiny."

The BBC added that it would "continue to capture... lessons learned on the programme".

EastEnders is currently watched by around six million people per episode, compared with about 7.5 million for its ITV rival Coronation Street.

The Manchester-based soap opened a new set in 2013, but the cost of that was never made public.

At EastEnders, the so-called "front lot", which is the replacement for the existing set, will now be finished by 2021 - but work on the "back lot", which will include new Walford outdoor locations, will go on until May 2023.

Work that has already taken place on E20 includes new boiler heating and edit suites, additional stage capacity and a new site entrance.

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