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News of the pending statement sent the pound lower by 0.3% against the US dollar to trade at 1.251, having traded higher by around 0.17% earlier in the morning - according to the Press Association.
Versus the euro, the pound slumped more than 0.4% to trade at 1.175, losing previous gains of more than 0.1%.
BBC assistant political editor Norman Smith says that without an official crest on the front of the PM's podium, it will be a Conservative Party announcement.
He says that would mean her stepping down or a snap general election are the likely options...
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The introduction of the Fixed Term Parliament Act by former prime minister David Cameron means that UK parliaments last for five years.
As a result, the next general election is not scheduled to happen until 2020.
However, the Prime Minister can call an election early if she gets the agreement of two-thirds of MPs in the House of Commons.
Theresa May only has a small minority as the governing party.
However, Labour leader Jeremy Corbyn has previously indicated he would support the vote.
Laura Kuenssberg
BBC political editor
Our political correspondents are at the scene and waiting for the announcement.
BBC assistant political editor Norman Smith says broadcasters usually know what is about to be announced, but on this occasion they do not.
He says Downing Street statements are usually of significance and can cover resignations and elections.
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The pound has taken a sharp dive at speculation - that Theresa May's speech could lead to a snap general election.
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Welcome to our live coverage of Theresa May's unscheduled announcement, which she is due to make at 11:15 BST. Speculation is mounting over what the prime minister will be saying.