Baltisse, an investment holding company belonging to Belgian entrepreneur Filip Balcaen, said in a post on social networking website LinkedIn that it had sold the Royal Park office building in the heart of Luxembourg City to an international investor.

Belgian newspaper De Tijd first reported the purchase and said Pontegadea was the buyer of the 10,000-square-metre (107,639-square-feet) building. The Zara founder's family office later confirmed the report to Reuters.

It is the first time Pontegadea has invested in real estate in Luxembourg, it added.

Ortega, who founded fashion giant Inditex in 1985, earlier this year bought a logistics centre in the Netherlands.

Pontegadea now owns $20.1 billion worth of real estate assets, having mainly invested in luxury buildings and office towers. It has also diversified its portfolio by buying energy and logistics assets in Europe and North America.

The Luxembourg complex is 95% leased to large law firms and private equity players, Baltisse said on LinkedIn.

($1 = 0.9239 euros)

(Reporting by Corina Pons; Editing by David Latona and Mark Potter)