Unite has informed EU commissioner for competition, Margrethe Vestager, that it intends to act as a third party opposing the acquisition of Air Europa by International Airlines Group (IAG).
The union is now in talks with the DG Competition case team who will be investigating the takeover.
Officials have strongly criticised a decision by British Airways owner IAG to spend hundreds of millions of pounds on a new acquisition, while pleading poverty as it reduces the size of its workforce.
United accuses British Airways of stripping workers of their terms and conditions while cutting thousands of jobs.
Sharon Graham, Unite executive officer, said: “Unite is questioning why IAG is seeking to fire and rehire the workforce at British Airways while it is ploughing ahead with buying an airline for over €1 billion.
“There are a number of reasons why this merger of Iberia with Air Europa would appear to be anti-competitive.”
She added: “The case team at DG Competition have been very open to us sharing our concerns with them.
“We are now gathering further detailed evidence from a number of expert sources and discussing the takeover with other parties that could be negatively impacted.
“We are confident that we have a strong case and we look forward to officially presenting all our evidence when phase one of the investigation begins.”
IAG agreed to buy the Spanish airline in November last year.
Unite has identified a number of serious competition concerns with the potential to scupper the Air Europa deal.
These include an increasing domination of the market for flights from Europe to South America by IAG, and the group’s dominant position at Madrid-Barajas airport.
These issues make it likely that the acquisition would drive up ticket prices and reduce the number of fights on some routes, matters of serious concern to the European Commission, Unite said.