A federal judge blocked the government Monday from requiring tobacco companies to begin putting graphic new warning labels on cigarette packages, cartons and advertisements next year.

In a 29-page decision, U.S. District Judge Richard J. Leon granted a request from five tobacco companies to hold off implementation of the Food and Drug Administration’s new mandate until the courts can review it.

The cigarette makers have “demonstrated a substantial likelihood” that their legal challenge would succeed and that they would suffer “irreparable harm” if the warnings were required while they challenge the constitutionality of the requirement, Leon wrote.

In granting the preliminary injunction, the judge wrote that the case poses a constitutional challenge to a “bold” effort by Congress and the FDA “in their obvious and continuing efforts to minimize, if not eradicate, tobacco use in the United States.”

At the very least, the unexpected decision will significantly delay the federal government’s most aggressive attempt to fight the nation’s leading cause of preventable death and the first major overhaul of cigarette warnings in a quarter century.

Tobacco companies hailed the decision. “We are pleased with the judge’s ruling and look forward to the court’s final resolution of this case,” said Bryan D. Hatchell, a spokesman for R.J. Reynolds Tobacco, the nation’s second-largest cigarette maker.

The American Heart Association, American Cancer Society, members of Congress and other anti-smoking activists criticized the ruling and urged the Justice Department to appeal.

“Judge Leon’s ruling ignores the overwhelming scientific evidence about the need for the new cigarette warnings and their effectiveness,” Matthew L. Myers of the Campaign for Tobacco-Free Kids said in a statement. “It also ignores decades of First Amendment precedent that support the right of the government to require strong warning labels to protect the public health.”

The FDA referred queries to the Justice Department, where spokesman Charles Miller would only say that officials “are aware of the court’s decision and are reviewing it.”

In statement issued late Monday night, the White House said:“We’re disappointed in this ruling. Tobacco companies shouldn’t be standing in the way of common-sense measures that will help prevent children from smoking. We are confident big tobacco’s attempt to stop these warnings from going forward will ultimately fail.”

The judge’s decision puts on hold a plan unveiled in June by the FDA to shock customers with nine graphic images of tobacco’s effects, including smokers exhaling through a tracheotomy hole, struggling for breath in an oxygen mask and lying dead on a table with a long chest scar.

Cigarette cartons, packs and advertising would have been required to feature those and six other graphic warnings, replacing the discreet admonitions that cigarette manufacturers have been required to offer since 1966. The startling images would have dominated half of the front and back of each carton and pack and 20 percent of each large ad.

The color images also would include a diseased lung, a mouth with mottled teeth and a disfigured lip, a weeping woman and a cartoon of a crying baby in an incubator along with messages such as “Warning: Cigarettes are addictive,” “Warning: Cigarettes cause cancer” and “Warning: Smoking can kill you.”

As part of his reasoning that tobacco companies were likely to win their First Amendment argument, Leon said the warnings clearly went beyond the government’s power to require companies to disclose information.

“It is abundantly clear from viewing these images that the emotional response they were crafted to induce is calculated to provoke the viewer to quit, or never to start, smoking: an objective wholly apart from disseminating purely factual and uncontroversial information,” Leon wrote.

Each brand would have rotated all the images randomly throughout the year. Every warning would also have to include “1-800-QUIT-NOW,” a hotline smokers could call for help in kicking the habit.

The FDA predicted that the images, which were designed to disgust and unnerve people of all ages, would reduce the number of smokers by 213,000 by 2013 and save $221 million to $630 million every year over the next 20 years.

Public health authorities and anti-smoking advocates hailed the new warnings as a milestone in the battle against tobacco in the United States that began in 1964, when the surgeon general first declared cigarettes to be a public health threat.

Cigarette makers began warning smokers that “Cigarettes May Be Hazardous to Your Health” in 1966. The warnings went through several modifications over the years but remained as small text on the sides of packages and ads.

Smoking rates steadily fell for decades but stalled in recent years, with one in five adults and teens still smoking. Smoking kills an estimated 443,000 Americans each year, costing nearly $200 billion in medical bills and other costs. President Obama himself struggled for years to quit.

The warnings are part of the FDA’s broad new anti-smoking strategy using powers to regulate tobacco in a 2009 law. The agency has restricted the use of the terms “light,” “low” and “mild,” banned the use of fruit, candy and spice flavorings and is considering taking action to prevent the sale of menthol cigarettes.