May has previously backed her predecessor David Cameron's pledge to cut the figure to the "tens of thousands", but there had been speculation in British newspapers the Conservatives might not include the promise in their manifesto.
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Earlier on Thursday a senior government minister and close ally of May told Sky News it was "not about putting numbers on it".
Net migration has consistently been running at around three times the government's target, with the latest figures in February putting the level at 273,000.
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When asked during a visit to a business in London on Thursday if she would include the number in the manifesto, she told the BBC: "We want to see sustainable net migration in this country, I believe that sustainable net migration is in the tens of thousands.
"Leaving the European Union enables us to control our borders in relation to people coming from the EU as well as those who are coming from outside the EU."
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