Hewlett-Packard Co. (HWP) won permission on Wednesday from antitrust enforcers to acquire rival Compaq Computer Corp. (CPQ) as the Federal Trade Commission closed its review of the combination.

The approval comes a day after the deal won a key endorsement from influential investor advisory firm Institutional Shareholder Services (ISS), which backed the proposal for computer and printing giant Hewlett's purchase of the No. 2 U.S. computer maker.

An estimated 10 percent of HP shares — a block that could determine whether or not the largest-ever computer merger succeeds — will be voted in accordance with ISS's opinion.

With about 20 percent of HP's shares already lined up against the deal — including those of the Packard Foundation and Walter Hewlett, son of the company's co-founder — ISS's support was viewed as critical to the merger.

The deal now awaits votes by shareholders of the two companies which are slated for March 19 and March 20. The families of the Hewlett-Packard founders have been lobbying against the combination.

"Completion of the FTC review marks a major milestone in the approval process, and we are now focused on winning the shareowner vote," Hewlett-Packard Chief Executive Officer Carly Fiorina said in a statement.

Shares of Palo Alto, California-based Hewlett-Packard closed down 41 cents to $20.18 while shares of Houston-based Compaq closed up 40 cents to $10.98 on the New York Stock Exchange.