With a budget of £215m ($361m, €270m), one might expect Tate Modern’s high-profile extension to be carried out to schedule and without incident. But a re-shuffle at the upper echelons of the project’s management, topped off with an admission that the budget will need to be revised, has raised questions from one of the UK’s Labour MPs.

“There’s £50m ($84m, €63m) of taxpayers’ money in this project which is late and going off track,” said Helen Goodman, the opposition’s minister for culture. She called on the Department for Culture, Media and Sport (DCMS) and Tate Modern to explain the situation after news of the high-level personnel changes.

Construction consultancy Gardiner & Theobald, which oversaw the first phase of the project, has been removed from its central role and replaced by developers Stanhope, according to an Architects’ Journal report.

Gardiner & Theobald’s new part in the 11-storey Herzog & de Meuron-designed extension is unclear, though a statement from the London gallery said the firm would still be “very much involved” at a senior level. Tate Modern said it was not unusual for responsibilities to change as large-scale development projects progressed.

Gardiner & Theobald announced the completion of the Tanks, the first stage of the development, in 2012. The transformation of the underground Tanks and the construction of the adjacent building will increase the size of the popular attraction by 60 per cent.

It’s not the first time Goodman has tried to pin down the government on details of the project. In April, she asked why the work was taking so long, and when the completion date would be. She was told by secretary of state for culture, media and sport Edward Vaizey: “The building has always been scheduled to open by the end of 2016 and remains on course to do so.”

“It is a large and ambitious project to establish a new model for museums of modern and contemporary art that will stand to the nation's benefit,” Vaizey added.

Apart from public funding, the work also received money from private donations, including £10m ($16.8m, €12.5m) from the Eyal Ofer Family Foundation.