THE Prime Minister has vowed to work to protect jobs at Bombardier in Northern Ireland after expressing bitter disappointment at a US decision to impose a potentially crippling tariff on the import of its passenger jets.

The US Department of Commerce has proposed a 220% tariff on the sale of the Canadian manufacturer's new C Series jets - an aircraft whose wings are made in Belfast.

The preliminary decision came after complaints by rival aerospace multinational Boeing, which argued that Bombardier received unfair state subsidies from the UK and Canada, allowing the sale of airliners at below cost price in America.

If the tariff is ratified in a final ruling, expected in February, it could have a devastating impact on Bombardier and, as a consequence, its Northern Ireland workforce.

More than 4,000 people are employed in Belfast by Bombardier and thousands more aerospace jobs in Northern Ireland are supported through the manufacturer's supply chain.

The Democratic Unionist Party (DUP), which the Tories rely on to swing key Commons votes for the Government, has repeatedly pressed the Prime Minister over the issue.

May, who has directly lobbied US President Donald Trump over the dispute, expressed regret at the decision.

"Bitterly disappointed by initial Bombardier ruling," a tweet on the Prime Minister's official account said.

"The Government will continue to work with the company to protect vital jobs for Northern Ireland."

While Bombardier branded the decision "absurd", unions have accused Mrs May of being "asleep at the wheel".

The controversial US decision came as Foreign Secretary Boris Johnson was using the launch of a new think-tank on Wednesday to push the cause for global free trade.

The future of the C Series appeared secure after Bombardier signed a 5.6 billion US dollar (£4.16 billion) deal in 2016 to provide the aircraft.

The manufacturer, which has been a major employer in Northern Ireland for 30 years, is due to begin delivering a blockbuster order for up to 125 new jets to Atlanta-based Delta Airlines in 2018.

Announcing the regulator's preliminary finding on Tuesday, US Secretary of Commerce Wilbur Ross said the subsidisation of goods by foreign governments was something that President Trump's administration "takes very seriously".

Union bosses warned that the preliminary determination was "unlikely" to be overturned by Mr Trump, who has been clear in his aim to fiercely protect American jobs - casting a shadow over the industry's future in Northern Ireland.

Ross Murdoch, the GMB union's national officer, said the initial ruling was a "hammer blow" to Belfast and risked sending shockwaves through Northern Ireland's economy.

"Theresa May has been asleep at the wheel when she could and should have been fighting to protect these workers. It's high time she woke up," he said.

Boeing's complaint arose after Northern Ireland's powersharing administration and the UK Government pledged to invest almost £135 million in the establishment of the C-Series manufacturing site in Belfast.

The programme also received one billion US dollars from the Canadian provincial government in Quebec in 2015 when its fortunes appeared to be ailing.

Boeing said it was seeking a "level playing field" for global competitors, but Bombardier accused its rival of hypocrisy.

ends Shadow industrial strategy minister Chi Onwurah accused the Prime Minister of "criminal negligence".

The MP for Newcastle upon Tyne Central told Sky News: "It is a damning indictment of the criminal negligence, complacency, of Theresa May and her Government.

"And it's a sign of their entire lack of international negotiating skills which bodes really badly for our post-Brexit negotiations.

"We wrote to the Prime Minister two weeks ago saying that she needed to speak out about this critical contract."

mfl Liberal Democrat leader Sir Vince Cable called on Mrs May to follow the example of her Canadian counterpart Justin Trudeau and "stand up" to the US.

He said: "Rather than simply expressing 'bitter disappointment', our Government should be out there fighting for vital British manufacturing jobs in Belfast.

"May must support Trudeau's administration in standing up to protectionist bullying from Donald Trump and his crude 'America First' philosophy.

"Do we really believe the US will save us from Brexit with a comprehensive trade deal, when this is how they deal with fair and free international competition?"