* Labour promises free fibre broadband for all
* Labour to nationalise BT's fixed line network
* UK PM calls the plan 'crackpot'
* Labour to tax Google, Amazon and Facebook to pay
* Labour suggests other telecoms could be nationalised
* BT shares fall as much as 3.7% (Adds Corbyn quote)
LONDON, Nov 15 (Reuters) - Britain's opposition Labour Party plans to nationalise BT's broadband network to provide free internet for all, a radical election pledge to roll back 35 years of private ownership that caught both the company and its shareholders by surprise.
The addition to Labour's already broad nationalisation plan for infrastructure sent BT's shares down as much as 3.7%, wiping nearly half a billion pounds off its market value.
But the market reaction was relatively muted as investors do not expect Labour to win the Dec. 12 election, analysts said. BT also retained the right to show UEFA Champions League soccer games, helping to support shares.
Labour said its proposed overhaul of the telecoms infrastructure would be paid for by raising taxes on tech firms such as Alphabet's Google, Amazon and Facebook and using its Green Transformation fund.
It would nationalise Openreach - the fixed-line network arm of the country's biggest broadband and mobile phone provider - as well as parts of BT Technology, BT Enterprise and BT Consumer to created a "British Broadband" public service.
"A Labour government will make broadband free for everybody," Labour leader Jeremy Corbyn said in a speech. "This is core infrastructure for the 21st century. I think it's too important to be left to the corporations."
"We'll tax the giant corporations fairly - the Facebooks and the Googles - to cover the running costs," said Corbyn, adding the public had been forced to pay far too much for "rip-off broadband" and the party's plan would transform the British economy.
BT, which traces its history back to an 1846 telegraph company, was once one of Britain's national champions and the flagship of Margaret Thatcher's privatisation policy when it was floated by her Conservative government in 1984.
Labour's announcement brought into sharp relief the election stakes: Prime Minister Boris Johnson who promises to deliver Brexit in January or Labour which says it wants to be the most radical socialist government in British history.
Johnson's Conservatives, who currently lead in opinion polls, said the broadband plan was a fantasy that would cost taxpayers tens of billions of pounds. Johnson has promised to roll out full-fibre broadband to all homes by 2025.
"What we won't be doing is some crackpot scheme that would involve many, many tens of billions of taxpayers' money nationalising a British business," Johnson said.
Labour said the cost of nationalising parts of BT would be set by parliament and paid for by swapping bonds for shares.
'VERY, VERY AMBITIOUS IDEAS'
In what would amount to the biggest shake-up in British telecoms since Thatcher's privatisations of the 1980s, Labour said few would lose out while millions would benefit.
The national Openreach network is also used by BT's rivals, including Sky, TalKTalk and Vodafone, to provide broadband to their own customers. Its only competitor with widespread coverage is Virgin Media, owned by Liberty Global.
TalkTalk said on Friday a deal to sell its FibreNation business had stalled after Labour's announcement.
Labour's second most powerful man, John McDonnell, suggested that the owners of the networks that compete with Openreach, such as Virgin Media's cable network and new fibre providers, could come to an arrangement or be nationalised too.
"We'll come to an agreement with them. It will either be an agreement of access arrangements, or working alongside us, or if necessary they can come within the ambit of British Broadband itself," McDonnell said.
Labour, led by 70-year-old socialist Corbyn, has been open about its plans to nationalise the rail, utility and water companies as well as to increase taxes on the wealthy, but has never previously suggested nationalising BT's assets and the company was taken by surprise.
"These are very, very ambitious ideas and the Conservative Party have their own ambitious idea for full fibre for everyone by 2025 and how we do it is not straightforward," Chief Executive Philip Jansen told the BBC.
Currently, fewer than 10% of British premises have access to full-fibre broadband - also called fibre to the premises - where fibre optic cable instead of copper is used to connect homes to the network.
BT has been criticised by customers, rivals and the regulator for poor service and a lack of ambition in upgrading its network to fibre, where Britain lags far behind European countries like Spain.
Labour said it would roll out the free broadband to all individuals and businesses by 2030, providing it to at least 15 million to 18 million premises within five years. It said the plan would save the average person 30.30 pounds a month.
There would be a one-off capital cost of 15.3 billion pounds to deliver the full-fibre network, on top of the 5 billion already promised by Johnson, the party said.
The Conservatives said the real cost would be over 83 billion pounds over a decade, that shareholders would sue the government and that it was unclear what the impact would be on BT's pension liabilities.
"It needs funding, it is very big numbers, so we are talking 30 to 40 billion pounds," CEO Jansen told the BBC. "If you are giving it away over an eight-year time frame it is a another 30 or 40 billion pounds. You are not short of 100 billion pounds."
Jansen said last month that Johnson's time scale for delivering full-fibre broadband to all homes would be extremely difficult to achieve.
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(Writing by Michael Holden and Guy Faulconbridge Additional reporting by Andrew MacAskill, Andy Bruce, Alistair Smout and Aishwarya Nair Editing by Rosalba O'Brien, Jane Wardell and Frances Kerry)