Thousands of Welsh jobs are at risk as Airbus has warned it could leave the UK if it exits the European Union single market and customs union without a transition deal after Brexit.
The European planemaker employs more than 6,000 people at its wing factory at Broughton in Flintshire and about 400 people at its base in Newport.
The Welsh Government said Airbus' warning was "extremely worrying".
"The UK cannot take the economic risk of cutting ourselves adrift," it said.
Airbus said the warning was not part of "project fear, but it was a "dawning reality".
Meanwhile Welsh Conservative leader Andrew RT Davies has sparked a row within his party after a statement from the Brexiteer accused the firm of making threats and doing a disservice to workers.
Many employees of the firm have "grave concerns" about the future, according to union Unite's convener for Broughton Darren Reynolds.
Speaking outside the factory, he said: "I have just been speaking with the members locally in the plant and they are really concerned now regarding what the future looks like for Airbus."
Mr Reynolds said he had visited Downing Street twice this year to discuss the impact of Brexit with government ministers but was not sure they understood the concerns.
He said: "This site here employs 6,500 people, it's key to this area and the north west to have the jobs that we do."
Many people from Wales also travel to Airbus' Filton plant in Bristol, which employs a further 3,000 people, where wings are assembled.
A further 2,500 work at sites in Portsmouth and in Stevenage.
It is estimated that the aviation giant supports about 11,600 jobs in Wales alone in the supply chain.
First Minister Carwyn Jones is holding talks with UK ministers and political counterparts at a meeting of the British Irish Council in Guernsey on Friday, where the issue is expected to be raised.
He has already issued a joint statement with Scottish First Minister Nicola Sturgeon urging the prime minister to pledge to stay in the EU's single market and customs union.
They have warned that Theresa May's plan for the UK's departure from the EU was not "consistent with the national interest".
In its Brexit "risk assessment" published on Thursday, Airbus said if the UK left the EU next year without a deal - meaning it left both the single market and customs union immediately and without any agreed transition - it would "lead to severe disruption and interruption of UK production".
"This scenario would force Airbus to reconsider its investments in the UK, and its long-term footprint in the country," it added.
The company, which makes all the wings for its passenger planes at its Deeside plant, also said the current planned transition period which is due to end in December 2020 was too short for it to make changes to its supply chain.
As a result it would "refrain from extending" its UK supplier base. It said it currently had more than 4,000 suppliers in the UK.
Analysis from Brian Meechan, BBC Wales business correspondent
This is not a shock in that Airbus has been warning for years that it might be less likely to invest in Wales and the rest of the UK in future if it doesn't have easy access to the EU.
It is a sign of its increasing frustration with the lack of clarity over the future trading relationship that it is now saying its long-term commitments, including to its Flintshire plant, could be in doubt.
Airbus currently has workers and products moving quickly and seamlessly across EU countries and it is planning for what happens if that stops after Brexit.
There are also a number of companies in Flintshire whose business in a large part revolves around supplying Airbus so there would be a ripple effect on the local economy if it invests less or begins to withdraw completely.
Some have argued that the company wouldn't really do this but many said the same about Tata's warnings over a number of years that it was at risk due to high energy costs and cheap Chinese steel.
Politicians only made moves to tackle its problems after it put its entire UK operations up for sale.
"Workers urgently require reassurance," said local assembly member Jack Sargeant, who has also called for an emergency statement in the Senedd.
The union Unite, which is the main union representing Airbus workers, said a 'cliff-edge' Brexit was becoming a "cold hard reality".
"It would be a betrayal of Airbus workers, their families and the tens of thousands workers in the wider supply chain if the government failed to secure frictionless trade and access to the customs union and single market," said the union's Steve Turner.
Responding to the comments from Airbus, a UK government official said: "We have made significant progress towards agreeing a deep and special partnership with the EU to ensure trade remains as free and frictionless as possible, including in the aerospace sector, and we're confident of getting a good deal that is mutually beneficial.
"Given the good progress that we are continuing to make in the negotiations we do not expect a no-deal scenario to arise."