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".
But Prime Minister Boris Johnson said it was "a crackpot scheme".
The plan includes nationalising parts of BT - namely its digital network arm Openreach - to create a UK-wide network owned by the government.
Mr McDonnell said the roll-out would begin with communities that have the worst broadband access, followed by towns and smaller centres, and then by areas that are currently well served.
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