A NATIONAL Audit Office update on HS2 has said that all parties ‘underestimated the complexity’ of building the high-speed rail network.

Forecasting a total overspend of up to 58 per cent, alongside a delay of up to seven years, the NAO report says that timescales for Phase 2b of the project through Cheshire remain ‘ambitious’.

Phase 1 of HS2, from Euston to Birmingham, is estimated to be up to £13bn overbudget and facing delays of up to a decade. Phase 2a, linking the west Midlands to Crewe, faces a £3bn overspend and a four-year delay.

Phase 2b could cost up to £41bn – 63 per cent above the original £25.1bn budget, with the estimated opening date pushed back from 2033 to as late as 2040.

The report said: “Despite being larger in scale than Phases 1 and 2a combined, Phase 2b has less funding allocated to it than the sum of the other two phases.

“HS2 Ltd estimates that it has completed around five per cent of the initial scheme design. The DfT has been planning to introduce legislation for Phase 2b into Parliament in June 2020. However, this is ambitious and any delays will impact on the planned opening date.”

A Department for Transport-commissioned report on the project by former HS2 chairman Douglas Oakervee is set to be published in the coming weeks.

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An independent review by his dissenting deputy chairman Tony Berkeley estimated a total spend of £108bn.

The NAO report added: “The scope of Phase 2 is uncertain. The outcome of the independent review of the programme may change the DfT plans for Phases 2a and 2b.”

Opinion remains split on HS2, with some 50 Conservative MPs calling on Boris Johnson to support the project while others across Parliament remain opposed.

Manuel Cortes, general secretary of transport workers’ union TSSA, said: “The dithering and speculation over HS2 needs to stop.

“The north of England has spent too long as the poor relation of transport investment. HS2 must go ahead and be built through to Scotland.

“It’s time we brought our railways into the 21st century with confidence and pride.”

The Confederation of British Industry's regional director for the north west, Damian Waters, added: “HS2 is an ambitious project and the NAO report usefully highlights the challenges of delivering large-scale infrastructure.

"But what is clear to the CBI and business generally, is the colossal cost of not delivering HS2.

“If the government truly believes in levelling up the regions, especially the north west, it should deliver HS2 in full.

Penny Gaines, chairman of campaign group Stop HS2, said: “HS2 was supposed to solve the north-south divide, but they don't know if it will.

“The timescale for building it is long bust and the expected timetable will only run far fewer trains than originally proposed.

"Rather than tinkering with the proposals, the Government should pull the plug on the scheme and cancel HS2 in its entirety immediately.

"At the very least they should halt all HS2 enabling works immediately, pay for all the homes and businesses they have already taken and stop the huge environmental wrecking ball that it currently going on."