A report by the National Audit Office (NAO), which reports on Government spending, claims that decisions made in recent years have used inaccurate analysis. It is particularly critical of the economic claims made for the planned HS2 high-speed rail line.

The NAO questions the Department for Transport’s original analysis, which claimed that HS2 would bring a £2.40 benefit for every £1 spent. The DfT has since reduced the benefit to £1.40. The NAO says: “The Department does not always recognise the limitations of its analysis nor scrutinise outputs to ensure that they make sense.”

The spending ‘watchdog’ adds that “the lack of arithmetic rigour leaves the DfT’s performance evaluations open to challenge” and that “its decisions have been made using unrealistic analysis”. The DfT later identified errors which meant that some benefits had been counted twice.

The NAO also says: “There remains uncertainty as to how much private-sector contribution the Treasury is seeking for HS2 and for which elements it wants others to pay.

"The department needs a better understanding of the wider benefits that transport investment brings in order to negotiate larger contributions for future programmes.”

Mick Cash, general secretary of the RMT union, said: “Due to the continuing shambles at the DfT, plus a chronic shortage of core in-house skills, a whole army of consultants, analysts, accountants and other private operators are ‘making a killing’ out of essential rail projects that are absolutely key to meeting the continuing surge in demand for rail travel.”

A DfT spokesman said: “We welcome the NAO’s findings about improvements in our sponsorship of major rail infrastructure programmes as part of our long-term economic plan.”