HS2 is over budget and behind schedule because the major rail project and Department for Transport underestimated its complexity and risk, the National Audit Office says.

A recent report by the government's spending watchdog claimed significant challenges remained to complete the programme - forecast to cost £106bn according to a leaked government-commissioned review - and deliver value for taxpayers.

The rail line, which would run to and from Euston Station, has seen Camden campaigners from Primrose Hill, Belsize and Regent's Park protest against HS2 over social, environmental and safety concerns.

In May, Euston homeowner Hero Granger-Taylor will bring legal proceedings against HS2 after the High Court granted a judicial review into the rail project's tunnel design.

National Audit Office head Gareth Davies said: "There are important lessons to be learned from HS2, not only for the Department for Transport and HS2 Ltd, but for other major infrastructure programmes.

"To ensure public trust, the Department and HS2 Ltd must be transparent and provide realistic assessments of costs and completion dates as the programme develops, recognising the many risks to the successful delivery of the railway that remain."

A decision on HS2, which transport secretary Grant Shapps last month told the Ham&High would come "once all the facts were in", is expected in the coming weeks.