British PM May asks Trump to help over Boeing's Bombardier challenge

* May tries to get Trump to help British jobs

* Britain seeks to get Boeing (NYSE: BA - news) to drop Bombardier (Toronto: BBD-A.TO - news) challenge

* May needs Northern Irish support to rule (Adds quotes from Canadian foreign minister about working with Britain)

By Elizabeth Piper and Guy Faulconbridge

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 (Swiss: BBDB.SW - news) 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) for their majority in parliament.

May raised the issue with Trump in a call this month. She (Munich: SOQ.MU - news) also plans to discuss the issue with Canadian Prime Minister Justin Trudeau when they meet next week, a source close to the matter said.

May's trip had been scheduled in advance, and there will be other items on the agenda, a Canadian official familiar with the matter told Reuters.

"You have to try everything," Canadian Foreign Minister Chrystia Freeland said on Tuesday when asked about the joint approach with Britain, adding that she had spoken to Foreign Secretary Boris Johnson on Monday.

"We are as one. We are closely allied and are making our case in very strong partnership," she said on the margins of a cabinet retreat in Atlantic Canada.

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.

"The UK government is working tirelessly to safeguard Bombardier's operations and its highly skilled workers in Belfast."

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 (NYSE: DAL - news) at well below cost price.

A spokesman for May said Bombardier's jobs were "of huge importance" to Northern Ireland.

A Boeing spokesman said on Tuesday the company would not comment on a meeting between heads of state.

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.

"We're not there yet, and the work still has to continue."

PLANE FIGHT

Bombardier makes the CSeries CS100 and CS300 state-of-the-art carbon wings at a 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."

Bombardier called the allegations absurd.

"Boeing's petition is an unfounded assault on airlines, the traveling public and further innovation in aerospace," a spokesman said.

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 (Sao Paolo: R2:EMBR3S.SA - news) . The European Union rejected the claim. (Additional reporting by Kylie MacLellan and Tim Hepher in London; Conor Humphries in Dublin; and David Ljunggren in St. John's, Newfoundland; Editing by Kevin Liffey and Chris Reese)