Britain will seek a clean break from the European Union, Prime Minister Theresa May said in a speech that eliminated any doubt her country would pursue a starkly different path outside the bloc, which for decades has been at the heart of attempts at continental integration.

The speech, long anticipated and rich with detail, was celebrated by Brexit advocates as an endorsement of their most fervent hopes for a full-scale liberation from the dictates of EU headquarters in Brussels. EU advocates countered that May was steering the country toward a potentially calamitous break-up, leaving Britain with the Donald Trump-led United States as a partner but with few true friends in Europe.

European leaders offered measured responses, suggesting that Britain was becoming more realistic about its prospects in the complex divorce negotiations to come. But they also maintained that the United Kingdom would meet resistance as it seeks to cherry-pick the benefits of the EU while throwing off the burdens.

There was no immediate reaction from the incoming American president, who set alarm bells ringing across Europe this week by signalling that he was indifferent to the future of the European Union - and expected more countries to follow Britain's path out.

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Whether that prediction proves accurate could hinge on whether May succeeds or fails in charting a new course - one she said would be independent of EU rules on immigration, trade and justice.

After refusing for months to give "a running commentary" on Britain's negotiating strategy, May offered the clearest indication to date of the country's departure plans, which were set in motion by last June's referendum on Britain's EU ties.

May said Britain wants to be "the best friend and neighbour to our European partners" but cannot be "half-in, half-out" of the bloc, which was born from the ashes of World War II and is designed to prevent future conflict by uniting Europe around a common economic and political project.

"We do not seek to hold on to bits of membership as we leave," she said.

She went on to reject preexisting models for quasi-membership that have been favoured by those seeking "a soft Brexit". Her remarks instead point to a jarring departure that transforms Britain's relations with Europe.

Britain, she said, will jettison membership in both the single market - which guarantees the free flow of goods, services and people across national boundaries - as well as the customs union, which dictates the terms of trade between Europe and the outside world.

Instead, she said, Britain will seek preferential trade access to European markets through a new agreement. And she said the country would strike out on its own in negotiating trade deals outside the EU.

Such a break has been widely anticipated, though never formally spelled out.