A BBC project to rebuild the set of EastEnders is expected to cost £27 million more than planned, sparking concerns over licence fee payers' value for money.
Reconstruction of the soap opera set is part of wider improvement works at BBC Elstree, which are set to cost the public broadcaster £86 million in total, with a completion date now two and a half years late.
Lack of expertise, over-optimism regarding costs and delays in construction have led to a 45% budget increase for the project, according to the National Audit Office.
Auditors concluded that the BBC cannot provide value for money on the wider project - termed E20 - which will see the 1984 set of the soap in Elstree rebuilt and expanded.
However, the report added: "As the (EastEnders) Front Lot construction only started in October 2018 and the Back Lot is at an early design stage, it is not yet possible to conclude on the value for money of the latest programme plan."
E20 has been beset by problems, leading to the massive budget increase on the project, which was expected to save the corporation almost £500,000 a year and allow filming in high definition.
Meg Hillier MP, chairwoman of the Committee of Public Accounts, said: "It is critical, given the BBC's funding pressures, that every pound of licence fee payers' money is spent effectively.
"It is concerning that the BBC has been unable to keep to budget and time on this project again, given my committee had already heard about the need to revise its scope in 2016."
The original set of EastEnders was intended to be used for two years, but has been in use for 34. This has led to problems with health and safety, continual investment in refurbishments, and the inability to shoot in high-definition due to the decaying set.
It was also hoped the move would ultimately save money after the BBC was set the goal of reducing costs by £800 million under the new charter.
In 2015 the rebuild - using real brickwork to create the set rather than facades - was estimated to cost £59 million, and the project was to be completed by August this year.
A budget was then approved for the increased amount to £62 million, and a delay of 26 months forecast in 2016.
The NAO report found that the BBC had "inadequate expertise" in construction projects, the EastEnders production team was not properly involved in the work on the set, and there was an 11-month wait to secure a construction contract.
The watchdog cited "over-optimistic initial estimates of costs", inflation, and delays on health and safety grounds such as dealing with asbestos.
The NAO concluded: "The BBC will not be able to deliver value for money on the E20 programme in the way that it envisaged in 2015. Disappointingly, some of the reasons for this were built into E20 at the outset and could have been addressed earlier."
A new EastEnders set is expected to be delivered in 2023. It will include a Front Lot replicating Albert Square and the traditional set, and Back Lot, made to look more like the streets of 21st century London.
The NAO report said: "E20 has been subject to ongoing scrutiny and reporting and, in the past 18 months, the BBC has made many improvements. The benefits of the programme still appear to be broadly achievable, albeit at a later date."
The BBC said in a statement: "The set of EastEnders was built in 1984 and only intended for use for two years. Over 30 years later, 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 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."