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 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.
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”.