Euro zone ministers to choose next chair; Spain, Luxembourg, Ireland mentioned

BRUSSELS (Reuters) - Euro zone finance ministers will start on Thursday a month-long process of choosing their next chairman to replace Portugal's Mario Centeno who will step down in July, with ministers from Spain, Luxembourg and Ireland among possible candidates.

The chairman sets the agenda at monthly meetings of finance ministers of the 19 countries sharing the euro, called the Eurogroup, and works out compromises on key policies.

The job goes to one of the sitting finance ministers for 2 1/2 years, a term that can be renewed.

The Eurogroup president also chairs the board of the euro zone bailout fund -- the European Stability Mechanism (ESM) -- which saved Greece, Ireland, Portugal, Spain and Cyprus from bankruptcy during the sovereign debt crisis.

The decision on the replacement for Centeno, who is stepping down because he is ceasing to be Portugal's finance minister, will be taken on July 9th, four days before Centeno's term ends.

The successful candidate must get the support of at least 10 out of the 19 finance ministers around the table. Each country has one vote, so the voice of the biggest, Germany, counts the same as that of the smallest, Malta.

The job is part of the broader horse-trading of top posts in the EU that seeks to keep a balance between positions taken up by centre-right and centre-left parties, the north and the south, east and west as well as large and small EU countries.

Euro zone officials have mentioned Spanish finance minister Nadia Calvino, a socialist, as one of the top candidates, and she has the backing of Berlin. But Calvino would not say on Thursday if she would throw her hat in the ring.

"Spain has not taken a decision yet. We will look at the options and decide," Calvino told reporters.

But officials also mention Pierre Gramegna from Luxembourg, a socialist like Calvino, and Ireland's centre-right minister Pascal Donohoe. Over the next weeks each euro zone government can put forward its candidate.

(Reporting by Jan Strupczewski; additional reporting by Christian Kraemer in Berlin and Belen Carreno in Madrid; Editing by Toby Chopra)