Theresa May has reportedly called on US President Donald Trump to intervene in a trade dispute involving Boeing following pressure from the Democratic Unionist Party.
Canadian firm Bombardier won an order in 2016 to supply up to 125 Series passenger jets to US airline Delta. The aircraft wings are made at Bombardier's Belfast plant.
But rival firm Boeing complained to US authorities that the deal would unfairly support from the Canadian and British governments, including a £113m loan from the UK.
Downing Street has been told that if the US Department of Commerce rules against Bombardier, it could jeopardise 4,500 jobs in Belfast.
This has led the British prime minister raising the issue in a call with Trump on 5 September, BloombergPolitics reported which followed the intervention of Democratic Unionist Party leader Arlene Foster, the Times reported.
May's government relies on votes from the DUP to pass legislation through the House of Commons.
Boeing also alleges Bombardier engaged in "price dumping" by agreeing to sell 75 of their planes for nearly £10.6m ($14m) below their cost price.
"Equity infusions from government coffers not only rescued the programme but have given Bombardier the resources it needs to aggressively target the US market," Boeing said.
Bombardier described the allegations as "absurd" and maintained that the government investments "comply with the laws and regulations in the jurisdictions where we do business," the BBC reported.
Business Secretary Greg Clark went to Chicago to meet Boeing's chairman, president and chief executive officer, Dennis Muilenburg.
A government spokesperson said it was working to safeguard Bombardier's operations and its workers in Belfast. They said that ministers across the government have communicated with Boeing, Bombardier and the US and Canadian governments.
"Our priority is to encourage Boeing to drop its case and seek a negotiated settlement with Bombardier," they said.