Negotiations between the government and the Professional Air Traffic Controllers Organization broke off early this morning, virtually assuring a nationwide strike at 7 a.m. today.

"Contract negotiations between PATCO and the Department of Transportation have broken off," Robert E. Poli, the air controllers' president, said at 2:25 a.m. after leaving the negotiations.

Federal mediator Kenneth E. Moffett said, "Mr. Poli determined that he was going to go on strike . . . it appears to be inevitable."

Asked if he would try to talk to both sides again today, Moffett replied, "I don't know what that means in light of the fact that there's going to be a strike.

"There's a lot of things that we have to find out because we haven't had a strike in the federal government in a long time," Moffett said. The first and last major walkout by federal employes occurred in 1970 when about 200,000 postal workers staged a nine-day strike.

Transportation Secretary Drew Lewis said the Justice Department will seek an injuction to prevent a walkout this morning.

Poki said the government's latest offer, which came about midnight, amounted to an extra $10 million in benefits. The administration had said earlier that it could only offer a $40 million package. a

Poli said the new offer was over a period of 36 months as opposed to the earlier 42-month proposal and was arrnged in such a way that the air controllers would wind up with less money.

The Reagan administration made it clear that there would be no negotiations during such a strike -- illegal under federal law -- and President Reagan also told Lewis there would be "no amnesty" for air traffic controllers who walked off their jobs.

Lewis told reporters at the beginning of an evening recess that no progress had been made, and he held out little hope for a breakthrough that would keep the air traffic controllers on the job.

PATCO officials interviewed here and elsewhere yesterday said Poli was certain to get a strike authorization vote in the membership polling, which started immediately after midnight on the East Coast. A union official said the results would be announced about 4 a.m. Washington time.

David A. Trick, PATCO director of operations, said the strike was inevitable. "There is no doubt that we are going to go out ," Trick said yesterday, and hour before the negotiatiors went into their first meeting at 2 p.m.

When that meeting recessed 3 1/2 hours later, at the request of union negotiators, the situation had not improved. Negotiations returned to the bargaining table at 9 p.m.

Last night, the Federal Aviation Administation ordered 150 military air traffic controllers to report to the nation's airports this morning.

Defense Secretary Caspar W. Weinberger said earlier in the day that the military is prepared to mobilize as many as 700 military personnel to help alert the "almost chaos that would ensure" from a strike. But he said on "Issues and Answers" (ABC, WJLA) that it would take three or four days' training to get the military personnel operating effectively.

Poli's response was a statement saying that the use of "unqualified military personnel" would "place the flying public in great peril."

More that 95 percent of PATCO's nearly 15,000 rank-and-file members last week voted to reject a tentative agreement reached under a strike deadline June 22, necessitating a new round of bargining and creating the current conflict, one that Reagan administration officials and many members of Congress said they are determined to win.

Reagan yesterday warned again that he would "come down with the full force of the Justice Department" if they made good on their strike threat. Lewis said the president told him during a briefing that the government would not negotiate during a strike and that there would be "no amnesty" for striking air traffic controllers.

Similar warnings were issued by Attorney General William French Smith, who said he would "prosecute to the fullest extent of the law" any controllers who walked off their jobs.

"It would be a very unhappy situation for us to have to prosecute them [but] we want to let them know before they take that action what the consequences of that action will be," Smith said on yesterday's "Meet the Press" (NBC, WRC).

Striking controllers could get a year in jail and be fined $1,000 for violating laws against walkouts by federal employes. The union, which has a $3.5 million "contingency fund," could also be liable for millions of dollars in civil and criminal penalties, including civil suits brought by affected private parties.

Some members of Congress, notable Sen. Jake Garn (R-Utah), threatened to seek decertification of the union if it goes on strike.

At issue in the dispute are both money and time. The government, thorugh Lewis, has offered the controllers a $40 million package that Lewis says could be arranged any way the controllers want it.

The controllers want what they say is a $500 million annual increase in wages and benefits. The government says the package actually would cost about $681 million, about 17 times what the Reagan administration was willing to offer late yesterday. The controller's proposals include a 32-hour workweek, as opposed to the current 40-hour week, and a more liberal retirement plan.

Lewis has called the demands "outrageous."

In the first session yesterday, Lewis said he would push the controllers to produce an offer that is "anywhere reasonable" and to extend the strike deadline another week.

Poli refused, as did last week when Lewis made a similar request.