"The shareholders have given the mandate for an initial public offering and the search for a lead manager for the deal," the source said.
Amadeus is controlled by private equity firms BC Partners and Cinven with 52.8 percent, Air France (AIRF.PA) with 23.14 percent and Iberia (IBLA.MC) and Lufthansa (LHAG.DE) with 11.57 percent each.
The source said the shareholders have yet to decide whether to tender part of their holdings in the planned offering or issue new shares.
Amadeus was delisted four years ago when BC Partners and Cinven bought their stake from the three airlines.
Earlier, Spanish financial website finanzas.com said Amadeus, worth 8 billion euros ($11.80 billion), had approved the offering of a 30 percent stake in the company.
A spokeswoman for Amadeus declined to comment on the operation.
