"I am absolutely clear that as we look to negotiate a trade deal with the United States of America, the National Health Service will remain as it is today. It will remain free at the point of use. The National Health Service is not for sale," May said speaking at the parliament.
The UK prime minister added that London would continue to "stand by the principles of the NHS."
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The United Kingdom is currently part of the EU single market but if London leaves it after Brexit, Britain will have to negotiate new trade deals with its partners, including the United States.
The NHS is currently under fire as lack of its financing resulted in heavy queues and long delays to treatment, which was dubbed as a health crisis by the opposition and watchdogs.