A £2bn project to open a road tunnel beneath the Stonehenge world heritage site in 2026 is being put at risk by uncertainty over how it will be financed, MPs have warned.

Ministers have claimed the scheme is affordable, deliverable and will improve Stonehenge by taking the sight and sound of lorries and cars away from the ancient monument and reducing traffic congestion.

But the public accounts committee (PAC), which scrutinises government spending, said the project was at the mercy of the forthcoming spending review, the timing of which is uncertain because of Brexit.

The committee flagged up concerns expressed by a senior Department for Transport (DfT) official, who told members when evidence was taken by the committee: “If we want this scheme to continue, and to continue in a sensible and efficient way to the current timeframe, we cannot wait another year, or however long it takes to carry out a spending review.”

The MPs’ report coincides with heated public hearings arranged as part of a six-month examination by a panel of five inspectors. In May, the National Audit Office said risks and uncertainty surrounded the plan for the two-mile tunnel.

The PAC’s conclusions are another blow to the project. The committee said: “Decisions on how to fund the project are on the critical path for the planned opening date of 2026 and are now at the mercy of the much-awaited spending review. Delaying those decisions further will jeopardise the opening date.

“If uncertainty about funding is not resolved before 2020, the timetable and viability of [the] Stonehenge tunnel project will be put at risk.”

The tunnel project was to be financed under the PF2 private finance initiative, but in October 2018, the chancellor, Philip Hammond, cancelled future private finance deals, including for Stonehenge.

The DfT said it and the Treasury remained committed to the project, with the latter releasing funding of £21.5m during 2019–20 to keep the scheme going. Transport chiefs do not expect to fund the project from the 2020-2025 road investment strategy, instead making the case for additional funding as part of the next spending review.

According to the PAC, Highways England must have funding certainty by the end of the year if it is to begin the commercial process of finding a construction contractor in time to keep to its 2026 target opening date.

“The timetable looks very challenging. We understand that between now and 2021, when construction is scheduled to start, as well as getting clarity over funding, Highways England must also obtain the necessary planning consents, negotiate with contractors to find a preferred supplier and complete the detailed design for the project,” the committee said.

There is also concern from the committee as to whether the project will be delivered within budget. “Given the department’s performance on other major projects in recent years, for example Crossrail, where costs have overrun to the tune of nearly £3bn, we are more cautious,” the MPs said.

Possible hitches cited by the PAC include finding new sites of archaeological interest during construction.

A DfT spokesperson said: “This government is committed to investing in the south-west and the A303 [including the tunnel] and A358 schemes [a linked project further west] are on track to be delivered to their respective timetables. Development work is continuing as planned.”