The roads and rail regulator said the investigation, which will run to April, would focus on the root causes of the current under performance and whether the second road investment strategy can be delivered as planned.

It said the formal investigation was being launched with 13 months remaining on RIS2 to allow time for corrective action to be taken before the end of the road period, should it be necessary.

National Highways has been hit by project start delays and soaring cost of delivery on projects.

As well as concerns about road delivery, it is understood industry whistleblowers have approached the regulator about irregular activities in subcontract awarding practices on some projects.

Feras Alshaker, director of planning and performance at ORR, said: “National Highways has generally delivered well for road users, but in recent months we have become concerned that its performance has dipped in a number of areas and issues are recurring.

“We are opening an investigation now to help ORR and National Highways achieve a common understanding of what is causing the current dip in performance, and whether it is still reasonable for us to expect the levels of performance set out in the second Road Investment Strategy.”

The ORR said it would also scrutinise whether National Highways has the right processes in place to deliver its agreed commitments by the end of the road investment period in March 2025.

The regulator first raise concerns in its annual assessment of National Highways’ performance, published in July 2023. It said then that it had identified a number of potential risks, such as delivery of its capital portfolio and asset management strategy.

At the time, ORR pointed out that with a more back-end loaded programme of work, the impact of future inflation and any further delays to construction of major enhancements would pose a risk to meeting National Highways’ forecasted efficiencies of £2.22bn needed to deliver the RIS 2 on budget.

Since then, ORR said those risks have materialised, and performance has dipped across several areas.

Last year plans to build a new road linking the M6 with south Lancaster were shelved due to soaring cost pressures.

The green light for the £1.3bn A66 Northern Trans-Pennine upgrade project has also been delayed with the Government putting off a decision on whether to grant development consent until early March.