M&E labour agency BMS has walked off the Wembley project in a row over payment, Building understands.
It is understood that about 250 BMS electricians subcontracted to Phoenix Electrical have left the project, halting crucial electrical works.
The row comes as Lee Compton, former Emcor director, takes over as managing director of Phoenix Electrical, which has been put under increasing financial pressure as a result of its Wembley contract and has been granted time and cost concessions by Multiplex.
Brian Ingram, former Phoenix joint managing director in charge of Wembley, is understood to have been demoted to deputy managing director. Further details of the firm's structure, and whether Compton has bought the majority shareholding in the firm, were unavailable as Building went to press.
A Multiplex spokesperson confirmed that BMS had left the project at the start of the week but said he was expecting Phoenix to find a way to carry on with its work.
He said: "It is an issue between BMS and Phoenix. We are expecting continuity of work from Phoenix. As it stands at the moment it has to finish the work."
It is understood that the BMS electricians were undertaking vital testing work on the stadium, without which it could not operate.
It is understood that cabling contractor CBL, of which BMS managing director Bernard Goodchild is a director, may also have left the project.
CBL hit the headlines in January when two of its other directors were arrested on charges of kidnapping.
Site sources suggest that many of the workers were dismissed by text message without notice, raising the possibility of employment tribunals.
Multiplex told the Australian stock exchange on Monday that it expected "substantial completion" of the stadium by June, but believed it was entitled until at least September to finish the project.
Phoenix Electrical was unavailable for comment.