The currency fell below USD 1.20 before recovering slightly today. It has lost about a fifth of its value since Britain voted in June to leave the EU.
May is due to disclose details of Britain's plan for leaving the EU in a speech tomorrow. Her office says she will call for a "global Britain" open to the world.
British media say she'll signal a willingness to take the UK out of the EU's single market in goods and services in order to gain control over immigration.
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