The complex, which is now complete, houses Unite’s regional headquarters, as well as a 170-bed hotel and 1,000 person conference centre. It is thought to have cost more than £90m.
Mr McCluskey had previously suggested it was “a smear” to question the project and last year said the hotel complex “will be a powerful resource for working people for many years to come, and will stand as a monument to Unite’s financial strength and commitment to its members”.
The project had been overseen by independent surveyors and architects.
The second inquiry – by accountants Grant Thornton – looked at contracts awarded to the union’s suppliers and companies which carry out work on behalf of the union.
The union’s executive discussed the reports on Monday but the police have advised against publication until their own inquiries are complete. The intention initially had been to publish.
A union source said “this is the opposite of a cover up”.
A Unite spokesman said: “The newly elected general secretary of Unite pledged to find out if there was wrongdoing.
“She is committed to doing all in her power to recoup any monies lost, by all means necessary and holding anyone responsible to account.
“It is clear this transparency would not have happened without the determination of the general secretary to ‘leave no stone unturned’ to get to the truth.”
Labour’s shadow chancellor Rachel Reeves said: “I haven’t seen any details on it, but full respect to the Unite general secretary for doing these investigations and handing them on to the police.”