Media playback is unsupported on your device Media caption The history of Bombardier in Northern Ireland

Arlene Foster and Michelle O'Neill have written a letter to the US Vice President, Mike Pence, about the Bombardier trade dispute.

The company is facing a complaint from rival firm Boeing that it has engaged in anti-competitive practices.

In the letter, the DUP and Sinn Féin leaders say that if the case succeeds it would have "serious implications" for Bombardier's NI operation.

Image copyright Reuters Image caption The letter does not ask Vice President Mike Pence to put pressure on Boeing

Bombardier is a Canadian firm but is NI's largest manufacturing employer.

Boeing says Bombardier is selling its CSeries jet below cost in the US, and wants the US trade authorities to impose financial penalties.

The former first and deputy first ministers say that: "For a small economy such as ours, the significance of the contribution that Bombardier makes cannot be understated.

"The threat facing us, as a result of the ongoing case, is alarming and goes much wider than it may immediately appear."

The letter does not directly ask the vice president to put pressure on Boeing or the US Department of Commerce.

Instead, it states "we would ask you to give consideration to the implications any decision may have here".

A Canadian senator has described the multi-billion pound aircraft trade dispute that could threaten hundreds of jobs at Bombardier in Belfast as a "red herring".

The firm won an order to supply 125 C-Series passenger jets to US airline Delta last year.

Image copyright Pacemaker Image caption The wings for the C-Series planes are made in Belfast

Its rival, US company Boeing, has claimed that the move was unfairly subsidised by the Canadian state.

Liberal Party Senator Colin Kenny described the complaint as bizarre.

Speaking to the BBC's Good Morning Ulster programme, Mr Kenny said: "There isn't a single aircraft company in the world that doesn't have some form of assistance from government.

"Boeing is a big boy - it can take care of its interests by building a better plane at a lesser cost, I don't see why they're turning to their government now."

The wings for the C-Series jets are manufactured at Bombardier's Belfast plant, supporting hundreds of jobs.

The centre fuselage for both the CS100 and CS 300 are also assembled in Belfast.

Boeing has also complained about a UK government loan made to the Bombardier plant in Belfast.

On Tuesday, it emerged that Prime Minister Theresa May had phoned US President Donald Trump in relation to the dispute.

She is due to meet her Canadian counterpart, Justin Trudeau, in Ottawa to discuss it on 18 September.

'Toss of the coin'

Mr Kenny said he was "delighted" to see Mrs May's intervention to try to resolve the issue.

"Today, it's Canada, but tomorrow it'll be the UK and it'll be another company after that," said the Canadian politician.

"One thing we know for certain about President Trump is that he is not very predictable."

Mr Kenny said the United States was important to the Canadian economy, but he expressed concerns about how President Trump might respond to Theresa May.

"How the US behaves is awfully important to us - but it's a toss of the coin as to whether he would be receptive to Prime Minister May, or whether he would brush her off."

The US Department of Commerce is due to make a ruling later this month.

It could hit Bombardier with punitive tariffs.

Image copyright Getty/AFP Image caption In 2016 Bombardier won an order to supply up to 125 C-Series passenger jets to US airline Delta

Boeing has alleged that Bombardier engaged in "price dumping" by agreeing to sell 75 of their planes for almost $14m (£10.6m) below their cost price.

The company said it had appealed to the International Trade Commission "to restore a level playing field in the US single-aisle airplane market".

"Boeing had to take action as subsidised 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," it said in a statement.

Bombardier has described the allegations as "absurd" and said the government investments "comply with the laws and regulations in the jurisdictions where we do business".

Senator Kenny said Canada had "made it clear to Boeing" that it was unhappy with the aerospace firm, and said the Canadian government is now reviewing all of its purchases from Boeing.

"We were preparing to buy 18 new super hornet fighters, now we're taking a hard look at a number of super hornet fighters from Australia.

"We're considering whether we just buy them from Australia - no cash would go to Boeing then."