The Government and HS2 Ltd knew at least as early as 2016 that the new high speed railway line was over budget, it has been revealed.
Despite years of assurances from ministers and HS2 bosses, documents seen by the BBC show that project costs were spiralling above the £56bn budget even before MPs had signed off the first phase.
In May 2016, then chancellor George Osborne was told by Patrick McLoughlin, the transport secretary at the time, that the first stretch of the railway was already a billion pounds over budget.
This could be the tip of the iceberg with the documents raising the prospects of delays to the delivery and significant cost rises.
Phase one was due to open at the end of 2026, with the second phase scheduled for completion by 2032-33.
Some estimates now run to £100bn and years of delays for the complete package of works, including the first phase from London to Birmingham and the two legs of the second phase taking the line north to Manchester and Leeds.
The Department for Transport said: 'Like all major, complex projects delivery plans evolve over time.'
The news comes after prime minister Boris Johnson launched a review into the costs and benefits of the project with a decision on whether to scrap the scheme or, if not, how to proceed due by the end of the year.
The prime minister's review team includes Douglas Oakervee, a former chair of HS2 Ltd, with HS2 sceptic Lord Berkeley as his deputy and supported by a panel of experts.
HS2 was at the heart of Mr Osborne's original Northern Powerhouse plans and the former chancellor's own thinktank - established to push the powerhouse agenda - is also undertaking its own 'pro-HS2' review.
In response Mr Osborne's Northern Powerhouse Partnership (NPP), which he has chaired since July 2016, announced a review 'to inform the Government-commissioned Oakervee review of HS2'.
In a statement it said its purpose is to offer northern perspective and ensure that the north properly push view infrastructure investment needed rebalance.
The NPP review will be overseen by a panel including Cllr Nick Forbes, leader of Newcastle City Council, Kevin Hollinrake MP, the co-chair of the Northern Powerhouse All Party Parliamentary Group, and Howard Bernstein, former chief executive of Manchester City Council.
Henri Murison, director of the Northern Powerhouse Partnership said: 'The Northern Powerhouse is of critical importance to the future of UK prosperity, and without improving connectivity it will be difficult to address the underlying reasons for our lower productivity.
'In the coming weeks, this group of distinguished leaders and experts from across the North will be addressing the key evidence to ensure that the importance of economic rebalancing is fully understood by those undertaking the Oakervee review.'
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