Ericsson is planning to close its research and development site in Coventry with the loss of 700 jobs, according to union Unite.
The site only opened in May this year and the union says the announcement came as a "bolt out of the blue" to workers.
Unite says it intends to fight the proposal as it would rob the UK of an important technological site.
National officer Peter Skyte said, "This is has come as a bolt out of the blue for the workforce and their families, who were looking forward to a promising future in this newly opened site.
"The former Marconi entity acquired by Ericsson a few years ago was among the top ten investors in R&D in the UK and successive governments have maintained the illusion that the country of ownership of a company is immaterial," added Skyte.
"But this decision by Ericsson to close the Ansty Coventry site and withdraw its R&D work from the UK shatters this illusion, throws highly skilled workers out of work, and robs the UK of key technological development vital for its future."