The deal is expected to be a test case of how far new European Competition Commissioner Margrethe Vestager will push telecoms operators for concessions in return for her allowing more consolidation in the sector.

Companies typically use oral hearings in Brussels to argue their case in front of a broader audience in the hope of getting a favourable decision.

The European Commission last Friday asked third parties for their feedback on concessions offered by Orange. Neither the regulator nor Orange provided details.

Commission spokesman Ricardo Cardoso declined to comment.

Antitrust lawyer Peter Alexiadis at Gibson, Dunn & Crutcher said he believed Orange's proposal should be sufficient to get the deal through.

He said from what he understood about the remedies put forward, including the sale of overlapping assets and an offer to give network access to other telecoms companies, this would be enough to satisfy the Commission's competition concerns.

Alexiadis said one element of Orange's concessions might not be necessary as the Spanish telecoms regulator was already mulling changes in the area of network access.

"While one can understand the anxiety surrounding the uncertainty about the future regulated access regime in Spain, there must surely be other mechanisms to get that part of the equation right."

National regulators from Spain, France, Britain, Italy and Romania were among those present at the hearing, underlining the keen interest across Europe in Vestager's thinking compared with her predecessor's relatively soft touch in previous deals.

Representatives from Vodafone, Swedish telecoms provider TeliaSonera and its Spanish unit Yoigo and Spanish mobile provider Masmovil were also at the event.

Keen to reinforce the economic rationale behind the proposed acquisition, Orange has hired consulting firm Charles River Associates, whose team at the hearing included Kai-Uwe Kuhn, the former chief economist for the competition unit at the Commission, one source familiar with the situation said.

The Commission was represented by its deputy director general for mergers, Carles Esteva Mosso, and director for the communication and media sectors, Guillaume Loriot. The hearing is scheduled to end at 1830 GMT.

The Commission has set a deadline of April 30 to decide on the case.

(Reporting by Foo Yun Chee. Editing by Jane Merriman)

By Foo Yun Chee