By Jonathan Gleave

MADRID (Reuters) - Spanish builder ACS is reported to be on the verge of a deal which would let it partly divest its stake in infrastructure group Abertis and focus on its core energy investments.

A new joint venture, in which Luxembourg-based fund CVC would hold a 60 percent stake and ACS 40 percent, would buy ACS's 26 percent stake in Abertis, Expansion newspaper said on Monday, citing sector sources.

CVC and ACS declined to comment on the report.

The news was "positive for Abertis and ACS as it raises the speculative angle ... the sale, if confirmed, allows (ACS) to proceed with its strategy of deleveraging or raising its stake in Iberdrola," BPI said in a note to clients.

"Raising ACS's current 12 percent stake in Iberdrola to 20 percent would require some 2 billion euros ($2.66 billion) at current market prices, while the net cash in of the operation seems to reach 2.4-2.6 billion euros," the Portuguese bank said.

Based on Abertis's current share price, a 26 percent stake is worth nearly 2.6 billion euros.

The Spanish constructor has been seeking to divest non-core businesses such as infrastructure management and to focus more on the energy sector, in which it operates wind farms and is the largest shareholder in power utility Iberdrola.

On Friday, ACS announced the sale of the Spanish assets of its Dragados ports unit for more than 720 million euros, but most of the cash from the deal was erased by Dragados SPL's debt.

Formation of the ACS-CVC venture could be announced this week or next week, with the two companies putting 1-1.5 billion euros into it. The rest of the capital needed would come from bank debt, Expansion said.

Divesting its Abertis stake involves added difficulties since the infrastructure group is a flagship of Catalan investment, which ACS shares control over with the autonomous region's powerful savings bank La Caixa.

An earlier three-way deal between La Caixa, ACS and CVC to launch a debt-funded buyout of Abertis shares not held by the two main owners was reported to have been scrapped at the end of July, sending Abertis shares lower.

ACS said in a conference call after its first-half results in July that it was considering several alternatives for its Abertis stake and could announce a deal soon.

By 1105 GMT Abertis shares were up 2.7 percent at 13.455 euros and ACS shares were up 1.4 percent at 34.675 euros, compared with a 1.5 percent gain on Madrid's blue-chip IBEX index.

(Reporting by Jonathan Gleave; Aditional reporting by Greg Roumeliotis in Amsterdam; Editing by Sharon Lindores and Michael Shields)