The government has been accused of failing to go beyond “fluffy words” in its attempts to help more than 4,000 staff of aircraft maker Bombardier who face losing their jobs in an international trade dispute.

Workers from Bombardier’s Belfast plant will on Wednesday unfurl a banner outside the Houses of Parliament in London demanding that Theresa May and her ministers do more to safeguard their jobs.

Bombardier has been hit by a 300% import levy following a complaint from rival Boeing that the Canadian-owned company had dumped its C Series jets at “absurdly low” prices. Boeing claims that aid from the UK and Canadian governments amount to illegal subsidy.

The workers will demand that May summon Boeing executives to a summit with the Canadian prime minister, Justin Trudeau, and the unions.

Steve Turner, the Unite assistant general secretary, said: “The British government has a duty to defend UK manufacturing jobs against the bullying behaviour of Boeing.

“A failure to do so will signal that any ambition ministers have for a coherent industrial strategy is effectively in tatters and that they are happy to put Trump’s ‘America First’ policy ahead of UK manufacturing jobs.

“Boeing’s case is without merit, a fact that Theresa May has herself admitted. May and her government need to be battling for Northern Ireland’s Bombardier workforce which makes some of the most technologically advanced wings in the world.”

Greg Clark, the business secretary, defended the government’s handling of the dispute in parliament on Tuesday. He said he had warned the Boeing chief executive, Dennis Muilenburg, that the action against Bombardier would jeopardise the aircraft maker’s chances of winning UK military contracts.

“To jeopardise that reputation and relationship by doing something that’s completely unjustified is something that I don’t regard as being in the strategic interests of Boeing and I said that in terms,” Clark said.

Only Boeing can withdraw the trade dispute, which has been filed with the US International Trade Commission.

Q&A How does state aid affect the Bombardier dispute? Show The US commerce department has upheld Boeing’s claim that Bombardier was able to offer its C-Series planes to US airline Delta for a vastly reduced price because of illegal state subsidies from Canada and the UK, violating global trade rules. Under international rules, unfair subsidies from governments can take the form of grants, loans, equity injections, tax breaks and production contributions, if they give a company or an industry an unfair competitive advantage over foreign rivals. In this case, Boeing claims that a US$1bn (£750m) bailout of Bombardier by the provincial Quebec government in Canada in 2015 unfairly enabled the firm to sell its C-Series aircraft in the US at below cost price.

The claim also relates to the Northern Ireland Executive and the UK government, which pledged to invest £135m in a new factory in Belfast where the wings for the C-Series planes are manufactured.

Clark said May had twice discussed the dispute with Donald Trump, and that several members of the government had reinforced their “serious concerns” with the US administration over the ruling.

Rebecca Long-Bailey, Labour’s business secretary, said she feared that Bombardier and its workers “are considered a fair price to pay for a post-Brexit trade deal with President Trump by this Conservative government”.

Ross Murdoch, a national officer of the GMB union, said: “The government needs to go beyond fluffy words about everyone uniting and telephone calls from Theresa May to Donald Trump.

“On the basis the US president has refused to intervene, perhaps the relationship is not so special after all. The priority here must be about giving reassurances and certainty to Bombardier workers now.

“The government needs to use whatever influence it believes it has to get Boeing to back off immediately. Bombardier employees shouldn’t be left in limbo until an International Trade Commission hearing, which most likely won’t be heard until February.”