London: The US was warned of the Japanese "surprise" attack on Pearl Harbour which propelled Washington to join World War II and changed the course of history, a de-classified memorandum has revealed.

The then US President Franklin D. Roosevelt described the Japanese attack on Pearl Harbour on December 7, 1941, as "a date that will live in infamy", a day on which the slaughter of 2,400 troops drew America into the Second World War.

Now, on the 70th anniversary of Japan's devastating bombardment of the US Pacific Fleet at Pearl Harbour, Hawaii, the memorandum showed that President Roosevelt was warned three days before the attack that the Japanese empire was eyeing up Hawaii with a view to "open conflict".

The information, contained in a de-classified memorandum from the Office of Naval Intelligence, only adds to proof that Washington dismissed red flags signalling that mass bloodshed was looming and war was imminent, 'The Sunday Telegraph' said.

"In anticipation of possible open conflict with this country, Japan is vigorously utilizing every available agency to secure military, naval and commercial information, paying particular attention to the West Coast, the Panama Canal and the Territory of Hawaii," the 26-page memo said.

Dated December 4, 1941, marked as confidential, and entitled "Japanese intelligence and propaganda in the United States", it flagged up Japan's surveillance of Hawaii under a section headlined "Methods of Operation and Points of Attack".

It noted details of possible subversives in Hawaii, where nearly 40 per cent of inhabitants were of Japanese origin, and how Japanese consulates on the US's west coast were gathering information on American naval and air forces.

Japan's Naval Inspector's Office was "primarily interested in obtaining detailed technical information which could be used to advantage by the Japanese Navy", it stated.

"Much information of a military and naval nature has been obtained," it stated, describing it as being "of a general nature", but including records relating to the movement of US warships.