A House of Commons Commission inquiry has found that “serious mistakes” were made in the award of the £30m-plus cladding contract for Portcullis House.

The inquiry was launched after the House of Commons was successfully sued by US cladder Harmon after the Parliamentary Works Directorate, which is responsible for the £250m project, picked a more expensive British rival for the contract.

In a written reply to a parliamentary question, MP Archie Kirkwood, representing the commission, said: “The action is still sub judice but I can tell honourable members that the inquiry concluded in the first place that serious mistakes were made in the handling of the fenestration contract, which exposed the House to liability.

“In future major projects, the Parliamentary Works Directorate should establish more clearly: (a) the roles and responsibilities of key members of the project team; (b) a project management process to include guidelines and control systems; and (c) lines of governance within a culture of professional and technical support; and that these recommendations should be taken into account in the current review of the Parliamentary Works Directorate.”

The inquiry was led by Sir Thomas Legg QC, who conducted the government inquiry into arms for Sierra Leone in 1998, and project management consultant Peter Bosworth.

The report into the contract award was completed last month. Harmon is awaiting a court ruling on whether it can receive an interim payment while the final amount it is entitled to is settled. A decision on the interim payment is expected in May.

The final amount of damages due to Harmon will be decided in a second trial, expected to take place next year. The company’s lawyers are understood to be pressing for £12m.