The project, proposed and implemented by Australia’s Liberal Party, was initially meant to cost $29.5 billion (£15.3 billion), but now NBN Co estimates costs closer to $51 billion (£26 billion).
Labour’s plans could well stray down the same path, as the party has already come under fire for its proposed costs.
John McDonnell projected the project would cost £20.3 billion over a decade, and more than £200 million in maintenance costs per year.
Philip Jansen, Chief Executive of BT, said actual costs could come close to five times greater than estimated.
Speaking to the BBC, he said costs would be “not short of 100 billion pounds.”