Federal regulators on Friday appealed a judge's ruling that allowed Whole Foods to proceed with its planned $671 million purchase of Wild Oats.

The Federal Trade Commission also asked the U.S. Court of Appeals for the D.C. Circuit to temporarily halt the merger during the appeals process.

The agency requested that the court issue the stay before 10 a.m. MDT on Monday. Whole Foods said Thursday it agreed with the FTC's request not to close the transaction before that deadline.

The agency opposes the combination of the nation's two biggest natural and organic foods stores, saying it would lead to higher prices and limited selection for customers. Whole Foods and Wild Oats counter that they're just a fraction of the larger supermarket industry, and companies such as Kroger and Safeway are increasingly stocking more organic and natural foods.

U.S. District Judge Paul Friedman in Washington issued a decision Thursday that lets the merger proceed. Whole Foods' attorney Paul Denis said he reviewed the judge's 93-page opinion and is confident it will withstand higher court review.

"The opinion is firmly grounded in both the facts and the law, and provides a detailed analysis of 'what is really happening in the marketplace,' " he said.

Boulder-based Wild Oats shares on Friday rose the most in more than five years on optimism that the deal will go through. Shares soared 18 percent to $17.92. Still, they are trading below Whole Foods' offer of $18.50 a share in cash.

The entire opinion was filed under seal to protect confidential company information. A version with redacted comments is expected to be made available next week.

Clifford Aronson, a lawyer for Wild Oats, read the entire decision and said the judge rejected the government's argument that the two supermarkets competed primarily with each other, rather than conventional supermarkets, and would raise prices if they combined.

Aronson also said that the opinion made no mention of one notorious aspect of the case: Whole Foods CEO John Mackey's anonymous stock message board postings that trashed Wild Oats' strategy during a seven-year period. U.S. securities regulators are looking into Mackey's remarks.

Friedman late Friday rejected the FTC's request for him to delay the closing of the merger during the appeal. Legal experts said that request was largely procedural and it would be rare for the judge to grant the request.

The real test is whether the appeals court agrees to the FTC's request to temporarily halt the merger during legal review.

The Court of Appeals for the D.C. Circuit tends to be conservative and business-friendly, said Melissa Maxman, an antitrust lawyer in Washington.

Under terms of the deal, both Whole Foods and Wild Oats can walk away from the merger after Aug. 31 without paying the $15.2 million breakup fee.

Wild Oats

OATS: Nasdaq

$17.92

+ $2.71

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