Sir Terry Morgan: We don't know what the final cost of HS2 will be

Sir Terry Morgan, who quit as chair of HS2 Ltd last year, urged Ministers to "be brave and get on with" delivering the controversial project, but admitted he had no idea whether it would be finished on budget.

HS2, which will carve through 45 miles of Staffordshire countryside, was initially expected to cost £32bn, but is now budgeted at £56.4bn.

However, opponents say by the time it is finished the public purse will have been emptied by more than £100bn.

Giving evidence to the Lords' Economic Affairs Committee on the economic case for HS2, Sir Terry said “nobody knows what the number is” when it comes to the final cost.

"I think the triangle of scope, cost and time - something has to give," he added.

Sir Terry said he remains convinced that HS2 will be "a huge game changer" for the Midlands, adding that he found criticisms of the project a source of frustration.

"We're making long term decisions that I am very confident will create huge value for the UK economy," he told the committee.

"It frustrates me sometimes. We make a big, brave decision, and we keep wanting to pull the tree up by its roots and have another look at it.

"I just think on these big infrastructure projects... be brave and get on with it."

Sir Terry also maintained that it was right to prioritise HS2 over "sticking plaster" improvements to local and regional train services, and claimed that HS2's main benefit would be its extra capacity rather than speed.

Transport Minister Nusrat Ghani also appeared before the committee. She insisted that HS2 will be delivered on budget and claimed the Government will not compromise over costs.

She said providing value for money for the taxpayer was a key aim when constructing the line.

"We are keen that HS2 Ltd is driving down costs as much as they can," she said.

"We are committed to the budget. It's not about compromises, it is about constant reviewing to make sure we have got good value for money."

HS2 bosses have faced criticism for allegedly misleading MPs over the costs of the project – claims which they deny – while a report by the National Audit Office found that property cost estimates along the route had increased significantly.

Work is already underway on Phase One of the route from London through Staffordshire to Birmingham, which has been budgeted at £27.18bn and is set to open in 2027.

Phase Two is split into two sections from the West Midlands to Manchester and Leeds, costing £3.48bn and £25.7bn.