Image copyright AP Image caption Donald Trump warned US firms against making "a very expensive mistake"

US President-elect Donald Trump says he will impose punitive taxes on US firms that move manufacturing overseas.

Mr Trump promised a 35% tax on products sold in the US by any business that fired American workers, and built a factory elsewhere.

Firms should be "forewarned prior to making a very expensive mistake", he said.

Mr Trump has promised to help blue-collar workers, as well as reduce taxes and regulations on businesses.

"The US is going to substantially reduce taxes and regulations on businesses, but any business that leaves our country for another country, fires its employees, builds a new factory or plant in the other country, and then thinks it will sell its product back into the US without retribution or consequence, is WRONG!" [sic], the Republican tweeted.

"There will be a tax on our soon to be strong border of 35% for these companies wanting to sell their product, cars, A.C. units etc., back across the border," he went on.

"This tax will make leaving financially difficult, but these companies are able to move between all 50 states, with no tax or tariff being charged.

"Please be forewarned prior to making a very expensive mistake! THE UNITED STATES IS OPEN FOR BUSINESS."

Image copyright Getty Images Image caption Mr Trump won the votes of working-class Americans by vowing to keep jobs from moving overseas

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Last week Mr Trump tweeted a warning to Rexnord Corp, an industrial supplier based in Milwaukee that plans to move a bearings plant and its 300 jobs from Indiana to Mexico.

"Rexnord of Indiana is moving to Mexico and rather viciously firing all of its 300 workers. This is happening all over the country. No more!" he fumed.

Experts have warned that the president-elect will face legal challenges if he tries to impose tariffs on specific companies without congressional approval.

Gary Hufbauer, senior fellow at the Peterson Institute for International Economics, predicted that the US courts would block such a move.

Mr Trump, who defeated Hillary Clinton in the poll, is due to be sworn in on 20 January.