MADRID — Spain’s leading electricity and natural gas company, Iberdrola, will buy the northeastern U.S. utility UIL Holdings Corp. in a deal valued at about $3 billion, the companies said Thursday.

The combined company will serve 3.1 million electric and natural gas customers in Maine, Massachusetts, Connecticut and New York.

The deal values UIL at $52.75 per share, including $10.50 to be paid in cash for each share, and also includes the assumption of about $1 billion in UIL debt.

Shares of Iberdrola USA will be listed in the United States and UIL shareholders will receive a share for each UIL share they own.

Iberdrola USA’s utilities will be Central Maine Power, Berkshire Gas, Connecticut Natural Gas, New York State Electric & Gas, Rochester Gas & Electric, Southern Connecticut Gas and United Illuminating.

Shares of Iberdrola SA were up 0.33 percent to 6.07 euros ($6.89) each in morning trading on Thursday in Madrid.

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