Rampant construction inflation, knock-on costs of delays from Covid and more stringent fire testing of materials are being blamed for pushing the project seriously over budget.

With restoration work around 60% complete, the project is now running £63m over budget with contingency funds used up.

Lendlease has asked for £29m immediate interim cash to complete some key works needed before a revised full and final cost estimate can be produced for the city council.

The extra funding will see the project budget rise to around £357m.

The project programme, which was due to complete in 2024, is presently pencilled in for 2026, although the city council said Lendlease would be pulling out the stops to bring the latest forecast completion date forward.

But the final cost and programme timeline will not be known until the start of 2024.

In a statement, the council said: “By the end of this year ongoing work to open up the Town Hall roof – with the scope for further discoveries being made about the condition of the building which may mean more extensive work than originally anticipated is required – will be far enough advanced to give much better certainty about final costs and timescales.

“A request for whatever further additional funding is needed to complete the project will then be made in January 2024. This would also be funded through borrowing.”

Deputy Council Leader Cllr Luthfur Rahman said: “The length and complexity of the project is such that it has been buffeted by some unprecedented challenges, the cost impacts of which are magnified because of the sheer scale involved.

“Nobody is pretending this has been easy but the end result will be something truly special, a source of pride and a remarkable asset for Manchester.”