Setting out her 12 priorities for divorce talks, May says Britain will not seek a Brexit deal that leaves it "half in, half out" of the EU.
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May was speaking to an audience of foreign diplomats and Britain's own Brexit negotiating team.
Britons' vote to leave the bloc has opened a huge number of questions about immigration and the future rights of the many EU citizens already living in the United Kingdom and whether exporters will keep tariff-free access to the single European market and British-based banks will still be able to serve continental clients.
May's government has come under fire from investors, businesses and lawmakers for so far revealing little about the future relationship she will seek when she begins formal divorce talks by the end of March.
The Brexit talks, which are expected to be one of the most complicated negotiations in post-World War Two European history, could decide the fate of her premiership, the United Kingdom and even the future shape of the European Union.