* United States imposes 220 percent duty on CSeries jets
* PM May says 'bitterly disappointed' by initial ruling
* Says will work to protect Northern Irish jobs
* Ally DUP says plant is critical for Belfast
* Britain says Boeing's position not justified (Recasts, adds details)
LONDON, Sept 27 (Reuters) - Britain is bitterly disappointed by a U.S. decision to slap duties on Bombardier jets, Prime Minister Theresa May said on Wednesday, promising to protect thousands of jobs in Northern Ireland that the ruling puts at risk.
The London government said Boeing's stance was unjustified and not the sort of position it would expect of a long-term partner.
May had asked U.S. President Donald Trump to help find a solution to a trade dispute between Bombardier and its U.S. competitor, but the U.S. Commerce Department on Tuesday imposed an anti-subsidy 219.63-percent duty on the jets.
Though a preliminary decision, it dismayed Canada, where Bombardier is based, and adds to pressure on May.
Her government is propped up by a small party in Northern Ireland and views the United States as its closest ally as Britain moves towards exiting the European Union in 2019.
"Bitterly disappointed by initial Bombardier ruling," May said on Twitter. "The government will continue to work with the company to protect vital jobs for Northern Ireland."
The ruling threatens 4,200 jobs at a Bombardier plant in the UK province that makes the new CSeries 110-to-130 seat jets.
Boeing, the worlds largest aerospace company, accuses Bombardier of dumping the CSeries jet in the U.S. market and says the aircraft is being unfairly subsidised by Canada, a charge Bombardier denies.
The U.S. Commerce Department said it imposed a 219.63 percent countervailing duty on Bombardier's new commercial jets after it made a preliminary finding of subsidisation.
Canada's foreign affairs minister Chrystia Freeland said Canada strongly disagreed with the anti-dumping and countervailing duty investigations into imports of Canadian large civil aircraft.
While juggling Brexit talks, May had lobbied the United States hard and intervened personally with Trump, who has made America First a theme of his administration, to get one of the titans of U.S. industry to back off.
But the decision increases domestic pressure on May just as she tries to kickstart divorce talks with the EU.
After losing her majority in a snap election in June, May is dependent on the Northern Irish Democratic Unionist Party (DUP) in parliament.
The DUP said it would fight to keep the Belfast plant, Northern Irelands largest manufacturing employer, open. Bombardier makes carbon wings for the CSeries CS100 and CS300 there.
"It is critical for Belfast," Jeffrey Donaldson, a senior lawmaker in the Democratic Unionist Party (DUP), told BBC Radio.
"This would have devastating consequences on our economy if the factory were to close and we're going to do everything we can to ensure that doesn't happen."
The row comes weeks after Boeing began construction of its first European parts manufacturing site in Sheffield, northern England.
Boeing says it employs 2,200 people in the United Kingdom, its third largest supply base after the United States and Japan. (Additional reporting by Elizabeth Piper in Brighton; editing by John Stonestreet)