Labour’s biggest union backer Unite is facing questions over how it spent £98m of members’ money on a hotel and conference centre.
Some Labour MPs are calling for an inquiry into the Birmingham scheme, which was originally estimated to be £57m.
Labour leader Sir Keir Starmer said it was “an issue for Unite to address”.
Unite says accountability was built into the scheme “at every stage” to ensure value for members’ money.
The complex, which is now complete, houses Unite’s regional headquarters, as well as a 170 bed hotel and 1,000 person conference centre.
The union has cited its determination to ensure employment practices on the site were up to Unite’s standards as one reason for the escalation in costs. It also decided to add an extra floor to the hotel and upgrade it to “four star plus”.
The union says the scheme will save its members’ money on hotel bills and conference facilities in the long run.
But some Labour MPs says the project’s costs spiralled out of control and are calling on General Secretary Len McCluskey to commission an independent investigation.
Bermondsey and Old Southwark MP Neil Coyle, a Unite member, said: “Given the vast sums of ordinary working people’s subs that have disappeared into this sink hole, only an independent audit can thoroughly work out what has gone so terribly wrong.”
Asked whether he backed an independent investigation into the project, Labour leader Sir Keir Starmer said he had not seen the full details.
But he added: “It is clearly an issue for Unite to address and, therefore, these questions need to be put to Unite on behalf of members, and that’s where that ought to lie.”
In a statement, Unite said: “Every step of the way, the production of this complex was overseen by independent surveyors and architects.
“Accountability was built into the process to ensure that at every stage of this development we got value for this union’s money.
“All this was overseen by our democratically-elected, independent 62-strong executive council.
“It is insulting in the extreme to infer that their oversight is at fault and should be somehow replaced by another body.
“This is the body elected by our members to keep this union properly run and accountable, and they do this superbly.”
In a statement issued by the union’s executive committee in January, Mr McCluskey said: “All Unite members can take pride in the wonderful building we have created in England’s second city.
“It 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.”
Mr McCluskey, who is due to stand down as general secretary next year, claimed “hostile media” had sought to “undermine and smear Unite” over the project.
The executive committee said the cost increase was partly down to ensuring working practices on the site met Unite’s standards.
Only directly-employed workers on “at least national pay rates and with union membership” were allowed to work there.
“This measure severely limited the number of possible contractors available to bid for work,” the committee said in the statement.