The National Audit Office has announced it plans to investigate the Lower Thames Crossing following a letter from Transport Action Network (TAN) raising concerns about the project's rising costs.

The head of the National Audit Office (NAO), Gareth Davies, said he anticipated the agency would ‘examine and report' on the planned tunnel between Kent and Essex, and that work to monitor the project ‘had already started', according to The Guardian.

In a letter to the NAO, TAN flagged concerns around a lack of published business case for the latest Thames crossing, despite government guidance requiring a Full Business Case to be produced for ministers to consider prior to financial sign-off of a major infrastructure project.

With the recently published third Road Investment Strategy (RIS3) seeing an additional £174m from the Department for Transport (DfT) earmarked for the project, the cost of the tunnel is now reaching £11bn.

When the preferred route was announced in 2017, the budget was initially estimated to be roughly £4.4bn, but subsequent increases have seen the costs for the project creep steadily upwards.

Rebecca Lush, roads campaigner at the Transport Action Network, told The Guardian: ‘At the autumn budget, the chancellor announced the "final tranche" of public funds for the Lower Thames Crossing. Yet now we find out that the DfT have bunged another £174m towards this privatised road project, whilst refusing to publish the outline business case.

‘The spiralling costs and secrecy have all the hallmarks of HS2, with LTC already costing more per mile than HS2. Whilst the government is nationalising the railways it is privatising our roads, demonstrating the utter incoherence in transport policy.'

A DfT spokesperson said: 'The NAO routinely reports on major government investments, and we would expect the same to apply to the Lower Thames Crossing.

'The cost-per-mile comparison to HS2 is misleading - these are entirely different projects with different objectives.

'Doing nothing at Dartford is simply not an option. Creating a new connection will reduce congestion, boost economic growth and establish a new strategic trade route between the ports of the South East, the Midlands and the North.'

An NAO spokesperson said: 'The Lower Thames Crossing is a significant programme and is of high parliamentary and public interest. The NAO is tracking activity on the programme, which will inform its decision on the right timing for audit work. A work in progress notice will be published on the NAO website once work has begun.'