MADRID, Oct 31 (Reuters) - The Spanish sovereign fund SEPI is evaluating acquiring a stake in the country's largest telecom company Telefonica, it said on Tuesday in a filing to the stock market regulator.

The analysis does not assume a decision on an acquisition, SEPI insisted.

The possible acquisition by the Spanish state comes after Saudi Arabia's telecom company STC built a 9.9% stake in Telefonica worth 2.1 billion euros ($2.23 billion) in September.

($1 = 0.9426 euros) (Reporting by Inti Landauro, editing by Pietro Lombardi)