The BBC has been criticised by a group of MPs for the way it has handled the redevelopment of the EastEnders set.
In December, the National Audit Office (NAO) revealed that the project had gone £27m over budget.
The Public Accounts Committee (PAC) said the BBC was "complacent" and the project "was flawed from the start".
In a statement the BBC said it "strongly rejected" the "notion that there has been any complacency in managing this project".
The PAC said the BBC should provide annual updates on the work to "demonstrate it now has a firm grip on the project's costs and progress".
The current set has been there since 1984, with the soap first airing in February 1985.
The project to build a new HD-ready Albert Square and expand the external EastEnders set was due to be completed by 2018, but won't be ready until 2023.
The building project is codenamed E20, after Albert Square's notional postcode.
The original 2015 forecast for the cost of the project was £59.7m, but the revised budget is now £86.7m.
The PAC chairwoman, Labour MP Meg Hillier, said the BBC had made "fundamental planning mistakes", adding there was an "apparent complacency" with which the BBC approached the project which "is entirely at odds with EastEnders' strategic importance to the Corporation".
The report from the PAC points out that the soap is a flagship programme for the BBC but is being "outperformed by its soap rivals, such as Coronation Street, and its overall audience has reduced as fewer people watch traditional linear TV".
It adds: "Therefore, it is important for EastEnders that the BBC completes E20 so that the programme is best placed to not only succeed but to also secure its long-term future."
"It was a serious error at the outset not to consider exactly what skills would be needed to see E20 through," said Ms Hillier.
"The resulting shortfall in key expertise set the tone for much of what followed."
The report said:
- The BBC failed to allocate enough money to manage the risks and contingencies of the project in 2015.
- It took over a year for a design change process to be agreed between the E20 team and EastEnders production teams.
- Contract negotiations took six months longer than planned, partly because of discussions about what bricks to use for the set.
- The BBC underestimated the scale and complexity of the project, including how it would age the new sets so that they either exactly replicate what viewers are used to, or look realistic where there are new locations.
- The commercial problems the BBC has encountered on this project demonstrate its ineffective sourcing and negotiating approach, which may have implications for other contracts.
Some of this criticism was also included in the NAO report, which 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. And inflation has also risen faster than expected in the construction industry.
In a statement, the BBC said they welcomed the PAC's "recognition of the importance of the E20 project to secure the long-term future of EastEnders", and pointed out the project has "already delivered many vital improvements at BBC Elstree Centre which help other programmes".
"However, we strongly reject the notion that there has been any complacency in managing this project. Like any building work of this scale, there have been challenges along the way including construction market issues beyond our control and working on a brownfield site.
"We have made improvements to the project and continue to keep it under close scrutiny."