The Enquirer understands that the balcony fabricator and installer has now left the job following a bust-up with main cladding contractor Lindner.
The balcony package is believed to be worth £3m and steelwork has been installed up to the 12th floor of the 43-storey tower.
One site source said: “It looks like there has been a dispute between Lindner and one of its subcontractors who have now left the job.
“I think they were about a quarter of the way through their contract programme.
“Hopefully they can come to an agreement because it will be a real problem to get another firm in at this stage of the job.”
The Enquirer understands that the steelwork firm underbid the job and is looking to broker a compromise deal with Lindner.
The site source said: “No-one wants delays on this job so it’s just a question of what can be sorted out.
“Something like this can delay a job by months if another firm has to be brought in.”
A Balfour Beatty spokesman said: ““The progress of the external cladding and balconies is progressing according to the agreed programme and we will continue to work with our supply chain partners to ensure that this remains so.”