The former sub-postmaster told the BBC that the Post Office's business model is "dead in the water".
The company is reported to have lobbied the regulator to allow it to hike bills by 40% and face lower fines.
Nearly two-thirds of callers to the tax authority had to wait longer than 10 minutes to get through.
It is predicted the huge factory will create 4,000 jobs and many more in the supply chain.
The firm never acknowledged the project publicly but had recruited about two thousands workers.