The scheme for the Welsh Government has been the subject of a long-running arbitration case and is expected to come in more than £100m over budget and two years late.

Costain moved its latest results announcement to September 14 while it waited for the latest decision which centred on the designs of a retaining wall on the job.

A company announcement stated: “The arbitrator’s decision found that responsibility for the design information rests with Costain and, consequently, the additional costs associated with the building of the retaining wall is not a compensation event under the Contract.

“Costain is disappointed by the arbitration award which reverses the ruling of previous adjudication awards.

“Although the arbitration award relates to the liability for the additional costs associated with the building of a specific retaining wall under the Contract, it has implications for the responsibility for design information under the whole contract and therefore Costain’s ability to recover these costs.

“Costain is in ongoing discussions with the Welsh Government to reach agreement on a final financial settlement and to seek recovery of costs under the Contract.

“However, on the basis of the uncertainty of recovery of such costs following yesterday’s arbitration award, and subject to reaching a final settlement, the Group’s half year results for the period to 30 June 2020 will include a charge to the income statement of c.£45 million to adjust the revenue recognised based on the level of cash received to date under the Contract.

“Costain will continue to fulfil the Company’s obligations under the Contract, with completion scheduled in 2021.”

The results are also facing a £49.3m charge over disputed payments on a gas station job.

Costain’s share price fell by more than 20% during early trading on Thursday morning.