Theresa May Brexit speech in Florence
UK Prime Minister Theresa May says she is 'bitterly disappointed' as appeal to US President Donald Trump over Bombardier falls on deaf ears. Jeff J Mitchell/Getty Images

UK Prime Minister Theresa May is "bitterly disappointed" by the US government's decision to slap duties of almost 220% on Canadian manufacturer Bombardier's C series aircraft, parts of which are made in Northern Ireland.

May took to Twitter on Wednesday (27 September) to say the UK will work with the Montreal-headquartered engineering and aviation firm to protect jobs, including some 4,000 posts in Northern Ireland.

May has a key alliance with the Northern Ireland-based Democratic Unionist Party (DUP) to support her minority government in Parliament.

Earlier this month, the PM reportedly called on US President Donald Trump to intervene in the trade dispute, involving Boeing, following pressure from the DUP.

Bombardier won an order in 2016 to supply up to 125 CS100 Series passenger jets to US airline Delta. The aircraft wings are made at Bombardier's Belfast plant.

The US Commerce Department on Tuesday ruled in favour of aircraft maker Boeing, which had alleged that Bombardier used "unfair government subsidies" to sell aircraft at artificially low prices.

However, Delta Air Lines, has argued that Boeing doesn't even make the 100-seat planes it needs.

With additional input from Associated Press