A note demanding Argentina remove its flag from the UK territory of South Georgia - ahead of the Falklands conflict - is to go on public display.
The note was only discovered in an old diary last month
The message - from the then Governor of the Falklands Rex Hunt - was radioed to members of the British Antarctic Survey to pass to the Argentines.
The Argentine military landed on the island, 800 miles from the Falklands, on 19 March 1982.
The National Army Museum will display the note in a Falklands exhibition.
The written version of the transmission was scribbled down by British Antarctic Survey plumber Neil Shaw on 20 March, after he and colleagues spotted the Argentine flag being flown by a party claiming to be scrap-metal workers.
Mr Shaw - who recently found the note while clearing out at home - wrote in his diary that they had taken the note to the Argentine ship, where it was read out to the captain.
The captain said they had clearance to come ashore from the Argentine government.
Mr Shaw has loaned it to the museum for its Task Force Falklands exhibition, which begins on April 2, the 25th anniversary of the Argentine invasion of the islands.
Governor Rex Hunt transmitted the message the day after the landing
The 0530 message from Mr Hunt said:
"SAT. MARCH 20th 1982. FROM THE GOVENOR [sic] OF THE FALKLAND ISLANDS. You have landed illegally at LEITH without obtaining proper clearance.
"You and your party must go back on board [their ship] the BAHIA BUEN SUCESO immediately and report to the BASE COMMANDER AT GRYTIVKEN for further instructions.
"You must remove the Argentine flag from Leith. You MUST NOT interfere with the BAS [British Antarctic Survey] depot at Leith.
"No military personnell [sic] are allowed to land in SOUTH GEORGIA. NO FIREARMS are to be taken ashore."
The note signals that it was already clear to the British government that Argentina's claim to the sovereignty of the islands posed a real threat.
Dr Alastair Massie, head of archives at the National Army Museum, said: "There are some documents which, almost by accident, survive down the years and evoke a response from later generations because of the momentous events with which they are associated.
"This note, the British response to Argentine aggression, marked the countdown to the Falklands conflict and perhaps the least expected war in history."