A forthcoming book has revealed Chile's military intelligence helped Britain during the 1982 Falklands conflict.
British troops raise the flag on the Falklands during the conflict
The book has threatened a possible diplomatic row between Chile and Argentina over the revelations of a secret alliance with the UK.
Chilean president Ricardo Lagos has forwarded parts of the book to the Argentine foreign ministry.
The book alleges Chile provided intelligence in return for half-price military aircraft.
The book, The Official History of the Falklands War, details the deal between the governments of Margaret Thatcher and General Augusto Pinochet, said the BBC's Chilean correspondent Clinton Porteous.
Extracts from it claim "the Chilean military provided key information on the movement of Argentine forces and other assistance, and in return were offered a cut-price deal on the purchase of military aircraft".
Mr Lagos said he received a letter and extracts of the book from British Prime Minister Tony Blair. He replied to Mr Blair and forwarded the extracts to the Argentines.
The Falklands were invaded by Argentina in 1982
Chile was officially neutral during the conflict, but the book claims it considered a border offensive against Argentina to draw military forces away from The Falklands.
Argentina and Chile both had military governments at the time and were "close to war", the BBC correspondent said.
The alliance between the UK and Chile remained secret, he said, because the Thatcher government did not want to be publicly associated with a military government known for human rights abuses.
After leaving office, however, Mrs Thatcher admitted General Pinochet had helped save many British lives during the conflict.
The operation to re-take the Falklands, which came after Argentina invaded in April 1982, cost 255 British and 655 Argentine lives.