* May tries to get Trump to help British jobs
* Britain seeks to get Boeing to drop Bombardier challenge
* May needs Northern Irish support to rule (Recasts, adds Bombardier and DUP comment)
LONDON, Sept 12 (Reuters) - British Prime Minister Theresa May has asked President Donald Trump to intervene in a dispute between Boeing Co and Canadian rival Bombardier to help secure thousands of jobs in Northern Ireland.
British ministers have also approached Boeing directly in an attempt to get the world's largest aerospace company to drop its challenge against Bombardier, which could endanger a factory that employs 4,500 people in the British province.
Bombardier is Northern Ireland's largest manufacturing employer and May's Conservatives are dependent on the support of the small Northern Irish Democratic Unionist Party (DUP) party for their majority in parliament.
May raised the issue with Trump in a call this month. She also plans to discuss the issue with Canadian Prime Minister Justin Trudeau when they meet next month, a source close to the matter said.
A U.S. trade court is due to give a preliminary ruling on Boeing's complaint on Sept. 25.
"Our priority is to encourage Boeing to drop its case and seek a negotiated settlement with Bombardier," a British government spokesman said in a statement.
"This is a commercial matter, but the UK government is working tirelessly to safeguard Bombardier's operations and its highly skilled workers in Belfast."
A spokesman for May said Bombardier's jobs were "of huge importance" to Northern Ireland.
May is likely to find it difficult to convince Trump, who has made 'America First' a theme of his administration, to get one of the titans of U.S. industry to back off from defending what it views as its trade rights.
But the DUP is certain to maintain its pressure on her.
"The engagement at governmental level with Boeing and with the U.S. has been significant over the course of the summer because this is pivotal to the Northern Ireland economy," DUP lawmaker Gavin Robinson told the Irish national broadcaster RTE.
"Were not there yet, and the work still has to continue."
Boeing this year asked the U.S. Commerce Department to investigate alleged subsidies and unfair pricing at Bombardier, accusing it of having sold 75 of its CSeries medium-range airliners to Delta Air Lines at well below cost price.
Bombardier makes the aircraft's state-of-the-art carbon wings at plant in Belfast.
"Boeing had to take action as subsidized competition has hurt us now and will continue to hurt us for years to come, and we could not stand by given this clear case of illegal dumping," Boeing said in a statement.
"We believe that global trade only works if everyone plays by the same rules of the road, and thats a principle that ultimately creates the greatest value for Canada, the United Kingdom, the United States, and our aerospace industry."
Bombardier called the allegations absurd.
"Boeings petition is an unfounded assault on airlines, the traveling public and further innovation in aerospace," a Bombardier spokesman said.
"We are very confident the UK government understands what is at stake and will take the actions necessary to respond to this direct attack on its aerospace industry."
Industry sources said Boeing was unlikely to back down in the case, which mirrors a wider row with Europe's Airbus over subsidies that it perceives as a strategic threat.
The row could also reopen a debate over Britain's own support for Bombardier in Northern Ireland. In 2008, the UK provided 113 million pounds in development loans plus other local aid for the production of CSeries wings, prompting a complaint from Brazil's Embraer. The European Union rejected the claim. (Reporting by Kylie MacLellan and Tim Hepher in London and Conor Humphries in Dublin; Editing by Kevin Liffey)