(Reuters) -- Objections to Express Scripts' $29 billion plan to buy rival pharmacy benefits manager Medco Health Solutions are accumulating as U.S. antitrust regulators weigh whether they have enough evidence to stop the megamerger.

The Food Marketing Institute, which represents the biggest grocery chains, sent a letter to the Federal Trade Commission on February 2 saying the merged company would be able to cut payments to supermarket pharmacies that already operate at very tight margins.

(We) ask the commission to bring an enforcement action to enjoin the merger, the association said in its letter to FTC Chairman Jon Leibowitz, which was seen by Reuters on Monday.

The supermarket heavyweights join consumer groups and community pharmacies that have come out against the deal. A group of state attorneys general are reviewing the merger.

A source familiar with the deal said that key people at the FTC believe it should be stopped but want to ensure they have adequate evidence to win a court fight to stop it.

A decision on whether or not to sue is expected by the end of February or early in March, the source said.

Shares of Medco fell 9 percent to $57.88 on Monday after Reuters reported the grocery chain letter and the inclination of key FTC officials to block the deal. Express Script shares fell 4.6 percent to $49.70.

The FTC said it does not comment on pending investigations.

Express Scripts spokesman Brian Henry said: We remain confident the merger will close in the first half of 2012.

Pharmacy benefits managers (PBMs) like Medco and Express Scripts are hired by insurance companies to handle prescription drug plans. They sometimes provide drugs by mail order, through their own pharmacies and by contracting with chains and independent pharmacies.

The deal, announced in July, would combine two of the three largest PBMs that are big enough to manage prescription drug benefits for large, nationwide companies. The third is CVS Caremark.

A Medco-Express Scripts merger would create an industry leader with nearly one-third of the market.

The FTC has been working actively to prevent increases in health care costs. The FTC in late January sued to block Omnicare's $441 million bid to buy rival PharMerica Corp.

Omnicare and PharMerica are the top two companies in the long-term pharmacy services sector, and the FTC said the combination would harm competition and allow Omnicare to raise the price of drugs for the frailest of the elderly.

More broadly, the U.S. government has taken an aggressive stance against mergers within the top tier of an industry.

The U.S. Justice Department sued last year to stop AT&T's $39 billion acquisition of discount rival T-Mobile USA. It also successfully blocked H&R Block, the largest U.S. tax preparer, from acquiring TaxACT software maker 2SS Holdings Inc.

A CHORUS OF SKEPTICS

The letter from the Food Marketing Institute, which counts Safeway, Giant, Wal-Mart and Supervalu among its membership, adds another opposition voice.

In early January, a coalition of consumer groups, including U.S. PIRG and the Consumer Federation of America, also urged antitrust regulators to stop the deal.

Further, a group of about two dozen state attorneys general are looking at the merger, and could challenge it on their own.

Express Scripts has said that the deal is good for the American consumer because the larger company would have more clout to negotiate discounts with drugmakers.

In a show of confidence that the deal will go through, Express Scripts announced on Monday that it was offering a second round of notes aimed at financing the $29 billion transaction.

Medco shares, however, have consistently traded at a discount to Express Scripts' $71.36 per share offer, reflecting investor caution over regulatory approval of the deal.

FEARING THE SQUEEZE

The Food Marketing Institute said in its letter that seven of the largest supermarket chains had met with FTC officials to ask that the deal be stopped but did not identify the chains.

If they are squeezed financially, grocery store pharmacies will be forced to cut back hours, end discounts for generics and stop giving free antibiotics and other promotions, said the letter, which was signed by FMI vice president Cathy Polley and regulatory counsel Erik Lieberman.

The merger will allow the dominant PBM to control approximately 40 percent of the overall prescription drug volume in the United States, said the FMI letter.

The FMI argued, in addition, that the PBMs would not likely pass along to consumers any cost savings they would achieve by squeezing supermarket pharmacies.

Small, independent pharmacies, under pressure from big chains, have also complained about the deal, saying that their reimbursement rates from the big PBMs were being reduced to the point that some felt they might not be able to stay in business.

(Reporting By Diane Bartz; Editing by Tim Dobbyn)

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