The four-star venue in Birmingham commissioned by Unite as a commercial venture has been valued at tens of millions of pounds below its construction cost.

The Aloft venue, with 195 bedrooms and space for 1,200 conference guests, has been valued at just £29 million, despite Unite spending about £112mn of members’ money on the project, according to union officials.

In 2021, there were questions of propriety about how the contract was awarded and a member of staff was accused of “misleading” the executive about an over-budget construction project.

A further £14m of missing money has also been identified by a KC-led inquiry commissioned by Sharon Graham, Unite’s general secretary. The independent report said the sum did not feature in the final accounts and that it remained “a mystery as to how and when this figure had been assessed and then presumably paid” to a contractor.

Unite officials confirmed that the case was “now with the Serious Fraud Office”, and said Ms Graham would “leave no stone unturned in finding out if there was any financial wrongdoing”.

An SFO spokesperson said: “In line with long-established practice to avoid prejudice to law enforcement activity, we can neither confirm nor deny any investigation into this matter.”