MADRID (Reuters) - Vodafone (>> Vodafone Group plc) could be interested in buying Spain's fourth-largest mobile operator Yoigo (>> TeliaSonera AB) once it is clear how tough the European regulator will be on tie-ups in the sector, the British firm's chief executive said.

Takeovers in Europe's telecoms sector, which is facing its fifth year of declining revenues, could be held back if competition regulators demand stringent concessions in return for approving deals.

Yoigo, owned by Sweden's Teliasonera (>> TeliaSonera AB), has been a potential bid target for some time, with France's Orange (>> ORANGE SA) and Spain's Telefonica (>> Telefonica SA) also considered possible buyers.

"We are obliged to look at everything, although for something like Yoigo we would need the European Commission to make its stance clear on current consolidation deals," Vodafone Chief Executive Vittorio Colao said at a meeting in Madrid, in comments confirmed by a spokesman on Wednesday.

Market players are waiting for the European Union to decide on Telefonica's proposed 8.6 billion euro (7 billion pounds) takeover of KPN's (>> KPN KON) E-Plus unit in Germany to see if the regulator demands concessions in return for approval. A decision is due by June 23.

Rivals such as German telecoms and Internet services provider Freenet (>> freenet AG) and United Internet (>> United Internet AG) say Telefonica should be forced to give competitors cost price access to its German network and sell some of its prepaid brands.

Vodafone last month agreed to buy Spain's largest cable operator Ono for 7.2 billion euros in a move aimed at creating a stronger challenger to market leader Telefonica (>> Telefonica SA).

Speaking after the publication of first-quarter results on Wednesday, Teliasonera CEO Johan Dennelind said the firm would need to consider other options for Yoigo if it could not reach a sustainable size in Spain through organic growth.

(Reporting by Robert Hetz; Writing by Tracy Rucinski; Editing by Erica Billingham)