Work was due to start in October but Wates is no longer on site following a period of value engineering and construction trials.
Wates was awarded the deal last May and has set-up a supply chain of subcontractors for the scheme which has attracted £19.4m in Lottery and European funding.
Phil Harrison, Managing Director, Wates Construction, said: “We have spent the past ten months conducting value engineering and processing sub-contracting tenders to establish the most cost-efficient means to deliver the work, alongside construction trials within the main mill building.
“Now complete, these detailed plans are being reviewed by the Flax Mill design team and project partners to determine the next phase of the project.”
English Heritage has also taken over the management of the scheme.
English Heritage Project Manager Tim Johnston said: “Wates have completed their value engineering and construction trials within the main mill building.
“These findings are being reviewed by the Flax Mill design team and project partners.
“English Heritage will be working with our stakeholders over the next few months to develop a revised schedule of works based on these findings. Once this has been completed we will have a clear timetable.”
Construction was originally due to be completed in early 2016 with the site scheduled to be open to the public by spring 2016.
The Flax Mill Maltings comprises seven listed buildings including the Main Mill, which was built in 1797 and is the world’s first iron-framed building and the forerunner of the modern skyscraper.
Alan Mosley, Chair of the Friends of the Flaxmill Maltings, said: “The programme of works has been delayed and the contractor has left the site.
“This has followed a series of setbacks as tenders for work have been returned and the programme timetable has been extended.”