The Serious Fraud Office is investigating the construction of a hotel and conference centre owned by one of the UK’s biggest trade unions, the BBC can reveal.

Unite the Union spent a total of £112m of its members’ money on the project in Birmingham.

The building has since been valued at just £29m, suggesting £83m has been wasted.

A KC-led inquiry commissioned by Unite’s general secretary Sharon Graham also identified a missing £14m which has been described as a “mystery” and does not feature in the project’s final accounts.

Unite has told the BBC the case is “now with the Serious Fraud Office” and Ms Graham, who took over as Unite general secretary in 2021, 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.”

The Birmingham project was intended to be an investment for Unite as well as saving the union money with hotel and conference costs.

Construction was completed in 2020 and development includes a four star 195 bedroom hotel, a 1,000 person capacity conference centre, as well as Unite’s regional offices.

Employment tribunal documents reveal the union believes its ruling executive council had been misled as to the true value of the project.

In 2022 South Wales Police searched the union’s London headquarters as part of a separate bribery, money-laundering and fraud investigation.

The force has told the BBC that the investigation is ongoing.

A Unite spokesperson said: “It is important to note that Sharon Graham has had to endure repeated attacks by those with much to lose since she launched these inquiries, from both inside and outside the union.

“These have been sickening and horrendous but she has remained determined to get to the truth.

“We are also pursuing legal claims to recover money lost to the union and the general secretary has put safeguards in place to ensure that such things can never happen again.”