Brussels is Europe's 26th largest airport, located in Zaventum just outside the city which houses the main European Union and NATO headquarters.

It serves as hub for Brussels Airlines, which is being fully taken over by Lufthansa (>> Deutsche Lufthansa AG).

The airport saw passenger numbers drop 7 percent last year to 21.8 million as a result of deadly attacks in March, which forced the closure of the airport for 12 days.

Australian Macquarie's infrastructure fund, which owns a 36 percent stake, is currently in talks with co-owner Ontario Teachers' Pension Plan), which has 39 percent, on whether the Canadian investor wants to increase its stake, the sources said.

Macquarie wants to exit as the fund in which it holds part of its stake has matured. Brussels airport is the last remaining asset of the Macquarie European Infrastructure Fund (MEIF) I, while the remainder of the stake is held in its MEIF III, which was closed to new investors in 2010.

If the negotiations fail to come to a successful end, an auction will be started to find a third party investor, the sources said, adding that JP Morgan has been tasked with overseeing the process.

Macquarie and JP Morgan declined to comment. Ontario Teachers, which manages around $130 billion (£104.2 billion), and the Belgian finance ministry, which oversees the state’s 25 percent shareholding, did not immediately comment.

Brussels Airport was forced to introduce increased safety measure last year after suicide bombers killed 16 people and injured over 150 in its departure hall in March, part of a coordinated assault on the Belgian capital's transport system.

Macquarie Airports (MAP) , a fund managed by Australian investment bank Macquarie Group Ltd, originally bought a 70 percent interest in the airport for 735 million euros (£621.8 million) in 2004 through a consortium vehicle. It increased its stake to 75 percent in 2007.

In 2011 it sold a 39 percent stake to Ontario Teachers as part of an asset swap in which it got the Canadian investor's stake in Sydney Airport stake.

Elsewhere in Europe, Ontario Teachers is looking to sell minority stakes in Britain's Bristol and Birmingham airports to take advantage of strong demand from pension funds and other long-term investors for the often-attractive returns on offer from such infrastructure assets, a source told Reuters this month.

(Additional reporting by Victoria Bryan; Editing by Maria Sheahan/Jeremy Gaunt)

By Arno Schuetze and Dasha Afanasieva

Stocks treated in this article : JPMorgan Chase & Co., Deutsche Lufthansa AG