The director of the National Park Service, after blocking Grand Canyon National Park’s attempt to ban the sale of small plastic water bottles, will now allow such bans, but under a restrictive set of conditions, the park service announced on Thursday.

Jon Jarvis, the director, issued a policy directive to all parks this week requiring that any park contemplating such a ban comply with a detailed checklist calling for written reports on issues like the amount of waste to be eliminated and the effect on the revenue of concessionaires and nonprofit groups that support the parks. The policy also requires parks planning a ban to consult with the park service’s public health office on its potential impact.

The decision reflects a tug of war within the park service over the idea of prohibiting plastic bottles, which make up a significant portion of park litter.

When Grand Canyon park officials were within weeks of putting a ban into effect a year ago, it was derailed by Mr. Jarvis, who told subordinates in an e-mail that the plan troubled representatives of the Coca-Cola Company, a major contributor to national parks.