BILLINGS, Mont. - Seven former National Park Service directors said in a letter released Monday that allowing the return of more snowmobiles to Yellowstone National Park would "undercut the park's resurgent natural conditions," and reverse air-quality improvements.

Yellowstone administrators are set to release on Tuesday their latest plan for snowmobiles in the nation's original national park. The issue of snowmobile use fueled years of legal wrangling. Pressure to allow more snowmobiles in the park has come from snowmobilers, nearby communities eager for winter tourists and the advocates' allies in Congress.

The intense debate reflects a broader struggle between those who want national parks preserved in a near-wild state, and those against restrictions on taxpayer-supported lands.

A draft version of the park's latest winter-use plan recommended allowing up to 720 snowmobiles a day. The cap would be in line with temporary rules in place the last three winters. Those rules expired last week.

During the three winters, actual snowmobile traffic averaged only about 250 machines a day, a sharp drop from the historical daily average of 765.

The letter from the former Park Service directors - sent to U.S. Interior Secretary Dirk Kempthorne - said allowing snowmobile numbers to return almost to historical levels would "radically contravene both the spirit and letter" of Park Service policies. The former directors claimed the service's own study on the issue supports their position.

"The study reveals that snowmobile noise would return to areas of the park where visitors are currently able to enjoy natural sounds and quiet," the directors wrote. "It demonstrates that exhaust would increase in Yellowstone's air."

Park Service spokesman David Barna said Kempthorne will respond to the letter in coming days. Barna added that a final snowmobile policy will not be set until after a public comment period.

The letter writers include all but one of the living, former Park Service directors who held the post since 1964. The years span the presidencies of Lyndon Johnson through Bill Clinton. The eighth living director, Fran Mainella, served under the current administration. Ethics rules prohibit her involvement because she resigned just last year, said a spokesman for the former directors.

Besides the limit on snowmobile numbers, the proposed winter policy would extend rules requiring that all snowmobilers be accompanied by hired guides, and that the machines have less-polluting engines.