The Interior Department's proposed changes in its strip-mining rules could lead to mining in 26 units of the national park system where it is now prohibited, according to the National Park Service.

In a strongly worded memorandum, the park service warned its parent agency that about 1.7 million acres of privately owned land within such national parks as Bryce Canyon in Utah and Denali in Alaska would be opened to strip mining if one proposed change goes through.

Congress "did not want surface coal-mining operations taking place on lands having an overwhelming national interest," wrote Richard H. Briceland, associate director of the park service for science and technology.

Interior is in the process of rewriting the government's stringent strip-mining regulations. Its Office of Surface Mining (OSM) has said the proposals are designed to ease regulatory burdens without harming the environment.

The memo came as a surprise to several Interior officials, who noted that Secretary James G. Watt has promised repeatedly to exempt the national parks from his plans for accelerated mining and drilling on public lands. But Watt also has placed a high priority on OSM's campaign for "regulatory reform."

"I get the impression that proposal is going to disappear," one Interior official said of the proposed change that would allow mining in the parks. Department spokesman Phil Million said the park service's comments "will be taken into consideration."

The proposal under fire would give mining companies broader rights to strip-mine coal on sensitive lands, such as national forests, parks and wildlife refuges. The current rules prohibit mining on these lands unless a company has bought rights to the coal beneath them and had obtained mining permits by Aug. 3, 1977, the day the federal surface mining act took effect.

Under the new proposal, companies would be required only to have acquired the coal rights, not to have obtained permits. This would allow mining on the 1.7 million acres within the 26 parks, according to the memo. It was reported earlier this year that the same proposal would open more than a million acres of national forests and wildlife refuges to strip mining.

The proposed rule change is one option for redefining a coal company's right to mine sensitive lands. The Park Service urged the OSM to adopt a different approach that would not affect the parks. Barring a court injunction, the final rules are scheduled to take effect by Nov. 15.

The memo said the proposed change might affect even more parks because it would allow mining on the periphery of other park lands.

The proposal was criticized yesterday by environmentalists. "This demonstrates the hypocrisy of Secretary Watt in charging ahead with mineral development at all costs on the public lands and yet proclaiming himself to be the guardian of the national parks," said Ron Tipton of the Wilderness Society.

The National Wildlife Federation, which sued OSM earlier this year in an attempt to block the rule changes, said the memo bolsters its contention that Interior has not revealed all the potential environmental impacts of its proposals.

Attorney Norman Dean said the federation plans to return to court to charge that OSM's environmental impact statement on the proposed changes is inadequate, a move that could delay implementation of the new rules.