AGL Resources cleared the last hurdle needed to buy Illinois-based Nicor Inc., a move that will double Georgia's largest natural gas distribution company and make it the nation's largest natural gas-only utility.

Illinois state utility regulators approved the transaction at a meeting Wednesday. The boards of both companies already have signed off on the $2.4 billion cash and stock deal, announced a year ago. At the time, the company said it expects the combined AGL to become a Fortune 500 company with an enterprise value of $8.6 billion.

AGL owns Atlanta Gas Light, five other regulated gas utility companies and several unregulated ventures. AGL has 2.3 million customers throughout its system, 1.5 million of which are Atlanta Gas Light customers. Nicor has 2 million customers.

"We are pleased with the [Illinois Commerce Commission's] decision to approve our merger," John Somerhalder II, chairman, president and chief executive officer of AGL Resources, said in a statement.

Somerhalder said AGL will review the commission's comments and plan to close the deal "in a timely manner."

The combined company's corporate headquarters and Atlanta Gas Light's operations will remain in Atlanta. The natural gas distribution arm that manages utility operations will be based in Naperville, Ill. The company has said Georgia customers should not expect any changes in their service.

The Georgia Public Service Commission did not have to approve the pending acquisition. However, a PSC decision in October 2010 paved the way for the deal to happen. The ruling said AGL and its investors could keep half the savings benefits from an acquisition for 10 years, instead of crediting the savings back to Atlanta Gas Light customers.