* Slim will invest 700 mln euros in indebted builder
* Slim to hold 25.6 pct stake, Koplowitz to retain 22.4 pct
* Investors welcome move despite steep share discount (Adds details on discount, earlier talks with Soros)
By Julien Toyer
MADRID, Nov 27 (Reuters) - Mexican billionaire Carlos Slim is set to become the largest investor in Spanish builder FCC after agreeing to buy top shareholder Esther Koplowitz's part of a $1.3 billion capital increase.
The deal means Koplowitz will lose control over the building group she inherited from her father, a German immigrant who built a construction empire during the years following Spain's 1936-1939 Civil War.
Slim's conglomerate Grupo Carso already holds small stakes in Spanish firms. But the investment of around 700 million euros ($870 million) in FCC, one of the Spain's top employers, is his highest profile move in the country.
Slim stepped in after talks between Koplowitz and George Soros broke down at the weekend.
In a statement to Spain's stock market regulator on Thursday, FCC said Slim's Inmobiliaria Carso would hold a 25.63 percent stake in the company following the capital increase. Koplowitz will see her stake reduced to 22.43 percent, from just above 50 percent now.
The arrival of a new core shareholder is good news for the group, which has lost more than 80 percent of its market value since the abrupt end of a construction boom in Spain in 2007 and needs to focus on rebuilding its business.
Unlike Soros, with whom Koplowitz had been in exclusive talks until Monday, Slim brings not only money but also industrial expertise in areas such as environment services and exposure to Latin America, where FCC wants to grow.
A source close to the talks said the discussions with Soros failed over the weekend when the financier tried to obtain a better price at which he would buy the shares.
"Koplowitz was in a weak position and he tried to push it a bit too far. That's when Slim stepped in," said the source.
The builder and services company is offering 43 new shares for every 41 held, at a price of 7.5 euros each.
Slim will acquire his shares at an average price of around 9.75 euros each, including the rights, a discount of 36 percent to their trading price of 15.96 euros at 1430 GMT or 14 percent below their theoretical ex-rights price (TERP) of 11.1 euros.
Yet shares in the firm jumped 5.3 percent after their trading resumed on Thursday following an earlier suspension, as investors expressed relief the indebted firm would stay afloat.
Slim and Koplowitz have agreed not to sell 85 percent of their shares for a minimum of 4 years, and they will each appoint four members to FCC's 12-strong board.
FCC, which has sold assets and laid off staff to focus on core activities and shrink a big debt pile, last week approved a 1 billion euro rights issue to help it pay down high-interest debt and cut costs. (1 US dollar = 0.8012 euro) (Additional reporting by Sonya Dowsett and Andres Gonzalez)