Unite wants Ford Bridgend meeting with bosses in Detroit
Union leaders have called for assurances over the long-term future of Ford's Bridgend engine plant.
Unite wants a meeting in the United States with senior executives with planned production of the new Dragon engine cut in half from 2018.
General secretary Len McCluskey said the scaling back of the investment was "extremely concerning."
The Welsh Government said "high level discussions" were planned to ensure the plant "continued to play a key role."
Mr McCluskey is hosting an emergency meeting of senior Unite Ford officials at the union's headquarters in London on Thursday.
Last week, the American car giant blamed changes in global demand for cutting back the planned investment in Bridgend from £181m to £100m.
The plant makes 500,000 engines a year for Ford's own cars but it is due to stop producing 250,000 engines for Jaguar Land Rover in two years' time.
But now, with plans to make only 125,000 of the new Dragon engines a year, unions say these figures suggest job numbers are "clearly" unsustainable.
A total of 1,850 workers are employed at Bridgend, which has been in operation since 1980.
Union officials said they feared last week there were attempts to run down the factory.
Mr McCluskey said: "It raises serious questions over Ford's long-term intentions for Bridgend and its commitment to its entire UK operation.
"Ford needs to provide answers fast if it is to head off speculation that it has a hidden exit plan for its UK sites.
"The Welsh Government has already backed Unite's calls for more transparency from Ford and it is now time for Theresa May and the Westminster government to do the same.
"Ultimately, the decisions around Ford UK are made around the boardroom table in Detroit.
"Today we are calling on the most senior Ford executives in the US to meet with myself and Unite to discuss these critical matters."
Ford had planned to build 250,000 of the new engines a year from 2018, but that figure has been halved.
On Wednesday, a joint statement from Economy Secretary Ken Skates and Unite's Wales secretary Andy Richards called for long-term jobs protection.
"Whilst changes in global demand create challenges, by working together with the company and by investing in innovation we believe that there is a way to ensure Ford keeps high quality production and good quality jobs here in Wales," they said.
Ford said it did not plan any job losses among the 1,850-strong workforce and said it still had a "substantial commitment" to the plant.
Welsh Conservative leader Andrew RT Davies said it was "great to see the trade unions stepping up to the plate" on the matter.
Criticising the first minister's efforts, he added: "Carwyn Jones was in America last week and should have moved heaven and earth to secure a meeting with the board in Detroit.
"But workers were instead left with a leadership vacuum - and an anxious wait for reassurances."