Longs Drug Stores Corp. has recommended that shareholders approve its $2.7-billion, or $71.50-a-share, purchase by Woonsocket-based CVS Caremark Corp. even as activist investor Bill Ackman hired a firm to explore getting a higher price.

The Longs board endorsed the takeover in a regulatory filing Monday.

California-based Longs also said in the filing that it held talks with another suitor that had offered $70 a share, but negotiations faltered over concerns about antitrust hurdles. Longs did not name the suitor, but several wire-service reports identified the potential bidder as Walgreen Co.

In June, after talks with the unidentified bidder stalled, Longs approached CVS about a bid, the filing said. The first bidder boosted its offer twice, to $70 a share, after the board sought a higher price. Longs’ board also was concerned that “regulatory issues” with the bidder could prevent the deal’s completion, the filing said

On July 11, the unidentified bidder said it was no longer interested in buying Longs because of “the uncertainty in California economic conditions and the regulatory risk” of the purchase, the filing said.

CVS then offered $71.50 a share. A purchase by CVS would increase its presence in California and other Western states, where Walgreen has more pharmacies.

“It’s a substantial premium to the stock prices a month before the announcement,” said Dan Poole, senior vice president of equity research at National City Bank in Cleveland, with $34 billion in assets including 3.54 million CVS shares as of June 30.

“CVS is the only logical buyer. We expect the transaction to be completed at the stated price,” said Dan Poole, senior vice president of equity research at National City.

Ackman’s hedge fund, Pershing Square Capital Management LP, has hired Blackstone Group LP to advise it, Ackman confirmed in an e-mail.

Ackman disclosed an 8.8-percent stake in Longs on Aug. 5, a week before CVS announced its planned acquisition. CVS’s $71.50-a-share offer was 32 percent higher than Longs’ close in New York trading that day.

Long’s shares surged 31 percent the day after CVS made its offer. CVS on that day fell 31 cents to $38.28.

Yesterday, CVS stock (CVS:NYSE) closed at $37.64, down 64 cents a share. Longs (LDG:NYSE) closed at $71.60, up 10 cents a share.