* Slim will buy into and guarantee a rights issue
* He is set to launch a full takeover bid after that
* Shareholders pact preventing full control will be revised
* Shares jump more than 12 percent on the news (Adds details, background)
By Carlos Ruano
MADRID, Feb 4 (Reuters) - Mexican billionaire Carlos Slim is set to launch a full takeover bid for Spanish building and infrastructures firm FCC after a planned rights issue due to be approved soon, a source familiar with the matter said.
Slim's holding company Inversora Carso group, is currently FCC's main shareholder with a 27.43 percent stake, closely followed by Esther Koplowitz, whose father founded the firm 70 years ago, with 22.44 percent.
But a 710 million euros ($794.92 million) cash call announced in December, which Slim has committed to buy into and guarantee, will likely push his stake above the 30 percent threshold that would require him to launch a full bid under Spanish regulations.
Last month Slim bought close to 2 percent of the company's share capital, according to filings to Spain's stock market regulator.
A source familiar with the matter said the only remaining stumbling block for the move to take place was now the shareholders pact that Slim and Koplowitz signed when the Mexican bought into a previous rights issue at the end of 2014.
Under the agreement, neither Slim nor Koplowitz could hold more than 29.99 percent of the capital for at least four years.
The source said, however, that the pact would now be revised in a bid to pave the way for the operation.
"The relationship between the two shareholders is extremely fluid and the pact will undoubtedly be modified," said the source on condition of anonymity.
A spokesman for FCC and Carlos Slim's Grupo Carso declined to comment. It was not possible to contact any spokesperson for Esther Koplowitz.
The move would make financial sense for Slim, who bought his initial stake at an average price of 9.75 euros per share and would now offer around 7 euros, the price at which he recently bought shares and that will be used as a reference if and when he launches a full bid.
It is unlikely that Slim will gain full ownership, however, because most shareholders are expected to buy into the rights issue - priced at 6 euros - while the subsequent bid at 7 euros may attract interest only from investors who bought shares below that level.
But his influence would rise in such a way that he would de facto fully control the company, which has been struggling with high debts and a sluggish business since a property bubble burst in Spain in 2007.
Having passed the 30 percent threshold he would also be in a position to keep building up his stake in the future without having to formulate any new bid.
Shares in FCC, which peaked at more than 61 euros per share in 2007, recovered to close to 16 euros in 2014 and were worth slightly more than 12 euros when Slim made its first investment in the firm.
The shares jumped 12.135 percent to 6.995 euros on the news.
Slim already made a similar deal at Spanish real estate company Realia, a unit of FCC on which he launched a full takeover bid on Jan. 28. ($1 = 0.8932 euros) (Additional reporting by Tomas Sarmiento in Mexico; Editing by Julien Toyer and Susan Thomas)