Nasa officials have admitted that they have received what is believed to be an artificial signal from space. The source of the signal is located over 2.3m light years from Earth and has been repeating on a loop in the narrowband frequency.
So far, there has been no confirmation of the location or the entire contents of the message. An excerpt of the message (see below) has been released ahead of a presidential address.
Leaders across the globe were briefed by the United States only this morning and are expected to make announcements shortly after President Obama delivers a televised address at 8am, 12pm (BST).
Downing Street said that the President spoke directly with David Cameron for 30 minutes last night on the content of the signal and agreed to co-operate on coordinating any response.
Speculation is rife as to what the President will say as White House and Nasa officials remain tight-lipped over many of the details. The speed of Mr Obama's address is being viewed as a response to several reports that news agencies were going to break the story in the next 24 hours.
CNN has reported unnamed Pentagon sources claiming that the transmission was detected two months ago, although this has not been confirmed.
Leaders around the world have urged people to remain calm until the message has been fully analysed by experts.
The Pentagon and Nato have raised their alert status while the Ministry of Defence said that it was keeping the situation under review, stressing that there was no direct threat to UK security.
Mr Cameron has briefed Nick Clegg and Ed Miliband with calls growing among MPs for an urgent statement in the Commons.
Part of the message believed to have been transmitted at 1420.356 MHz:
01101110 01100001 01101110 01101111 01101111 00100000 01101110 01100001 01101110 01101111 01101111 00101100 00100000 01000001 01110000 01110010 01101001 01101100 00100000 01100110 01101111 01101111 01101100 01110011 00100001 00001010
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AOL.co.uk Article - Nasa detects 'intelligent' alien signal