A man works on a C Series aeroplane wing in the Bombardier factory in Belfast, Northern Ireland September 26, 2017. REUTERS/Clodagh Kilcoyne LONDON — Senior UK politicians including the Prime Minister and her closest ally in Northern Ireland are involved in a diplomatic row with the United States over a new tariff that has thrown thousands of British jobs into doubt.

Prime Minister Theresa May, Defence Secretary Michael Fallon, and Northern Ireland First Minister Arlene Foster on Wednesday all criticised the United State's decision to place a big tariff on imports of Bombardier jets, putting at least a thousand Northern Irish jobs at risk.

The US Department of Commerce on Tuesday announced plans to levy a 220% tariff on every Bombardier C Series airliner imported into the US.

The ruling follows claims from rival Boeing that Bombardier's C-series planes were being sold into the US at abnormally low prices due to Canadian state subsidies, at the detriment of Boeing's own planes.

Bombardier is a Canadian company but has a significant manufacturing base in Belfast, Northern Ireland, where the wings of C Series aircrafts are manufactured. The factory employs around 4,500 people. Roughly 1,000 of those work on C Series planes and the tariff has led to speculation that jobs could be at risk.

'Bitterly disappointed'

Prime Minister Theresa May, who has previously been reluctant to criticise Trump's administration, said on Wednesday she is "bitterly disappointed" by the news.

Speaking at an event in Northern Ireland on Wednesday afternoon, UK defence minister Michael Fallon also said Boeing's behaviour in the trade dispute "could jeopardise" future contracts with the UK government, according to the Press Association.

US firm Boeing has lucrative contracts with the UK government, most recently signing a £306 million ($411 million) deal in June to build 38 Apache helicopters.

Nothern Ireland's First Minister Arlene Foster, who leads the country's Democratic Union Party (DUP) and supports May's minority government, said the move was a "very disappointing determination," but said it was "not the end of the process."

"There are further steps that will follow," she said in a statement.

The Confederation of British Industry (CBI), a lobby group which represents 190,000 UK businesses, warned on Wednesday that the new tariff will pose a "challenge" to Northern Ireland's economy.

"Bombardier makes a huge contribution to the Northern Irish economy," said Angela McGowan, CBI's Northern Ireland director. "They are a major regional employer and support a wider supply chain that stretches across Northern Ireland."

'America First'

US President Donald Trump was elected on an "America first" platform and has pledged to impose bigger tariffs upon foreign imports in a bid to boost American manufacturing.

The move raises the prospect of a trade war between the US and Canada.

The US International Trade Commission will issue a final judgment on the Commerce Department's proposed tariffs in early 2018.