The European Commission on Wednesday put off a ruling on Lufthansa's takeover of Austrian Airlines and opened an in-depth probe that could jeopardise the deal, which was opposed by rival airlines.

Competition Commissioner Neelie Kroes announced the decision in a statement, saying there were fears that the takeover deal would lead to an increase in prices for passengers and possible reduced operations on some routes.

A source close to the matter, speaking on condition of anonymity, told AFP earlier: "Lufthansa's offer for Austrian Airlines expires July 31 and if the conditions are not met then they will drop the offer."

Lufthansa wants to acquire AUA fully and signed a deal last year to acquire a 41.6-percent stake in AUA from the Austrian state holding company OeIAG for the symbolic sum of one euro cent per share or just over 366,000 euros in all.

One of the conditions for the deal was an agreement that the Austrian state would absorb 500 million euros -- or just over a third -- of AUA's debts.

But those terms have enraged rival airlines, such as Franco-Dutch carrier Air France-KLM, which had also sought to buy AUA, as well as other airlines such as British Airways and low-cost carrier Ryanair.