BT will be part-nationalised and every household will be offered free broadband if Labour win the election, they have said.
The announcement will be made by party leader Jeremy Corbyn, who plans to pay for the scheme by taxing multinational technology firms such as Amazon, Facebook and Google.
Mr Corbyn will make the announcement in a speech in Lancaster tomorrow, when he will describe the new free public service – dubbed British Broadband – as "central to Labour's plans to transform our country and economy".
It will first be rolled out in communities with the poorest broadband access in the UK, such as rural areas, before being expanded across the country.
Mr Corbyn will say the "massive upgrade in UK's internet infrastructure" will save the average person £30.30 a month.
There are more than 27 million households in the UK, according to the Office Of National Statistics.
Sky News' political correspondent Jon Craig said it was a "big moment" and a "huge and controversial pledge", adding that it was "going further than Labour have done before on renationalisation".
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He said there would be "strong reaction" to the announcement, which has already been dismissed by the Conservatives as a "fantasy plan" that would cost taxpayers tens of billions of pounds.
Digital, Culture, Media and Sport Secretary Nicky Morgan, who is standing down at the election, said: "Corbyn is clearly so desperate to distract from his party's divisions on Brexit and immigration that he will promise anything, regardless of the cost to taxpayers and whether it can actually be delivered. What reckless idea will be next?"