Labour has promised to give every home and business in the UK free full-fibre broadband by 2030, if it wins the general election.
The party would nationalise part of BT to deliver the policy and introduce a tax on tech giants to help pay for it.
Shadow chancellor John McDonnell told the BBC the “visionary” £20bn plan would “ensure that broadband reaches the whole of the country”.
The Tories said it was a “fantasy plan” that would cost taxpayers billions.
And the Lib Dems called it “another unaffordable item on the wish list”.
BT chief executive Philip Jansen told Radio 4’s Today programme Labour had under-estimated the price of its pledge.
But he said he was happy to work with whoever wins the election to help build a digital Britain, although the process for implementing Labour’s plan would not be “straightforward”.
He added that the impact of any changes on BT pensioners, employees, shareholders – and the millions of investors via pension schemes – needed to be carefully thought through.
However, TechUK, which represents many UK tech firms, said the proposals would be a “disaster” for the telecoms sector and customers.
Prime Minister Boris Johnson has promised £5bn to bring full-fibre to every home by 2025.
But Mr McDonnell said the Conservatives’ funding plan for improving broadband was “nowhere near enough” and would leave the UK falling further behind other countries who already have fibre more widely available.
Broadband packages in the UK cost households an average of around £30 a month, according to comparison site Cable – which people would no longer have to pay under Labour’s scheme.
The party claims it would “literally eliminate bills for millions of people across the UK”.
The BBC’s business editor, Simon Jack, said Labour’s proposal had caught BT “off guard”, as Mr McDonnell had said in July that he had no plans to nationalise the telecoms giant.
The company has disputed the cost of rolling out fibre broadband to every home and business, saying it would cost closer to £40bn than £20bn.