President Reagan is increasingly concerned about the possibility that the Soviet Union will use force against Poland's labor unions, and he warned in a statement yesterday that nay external intervention in Poland or suppression of the Polish people "could have a grave effect on the whole course of East-West relations."

Reagan's warning came as the State Department said the Soviets are extending military maneuvers within Poland and near the Polish border as an apparent "warning flag" to the Polish labor unions.

Reagan's statement not only was a deliberate effort to caution the Soviets against use of force but also was new evidence of the White House role in foreign policy after the two-day, heaadlined clash in which the senior White House advisers won a victory over Secretary of State Alexander M. Haig Jr.

The statement, issued by press secretary James S. Brady, but labeled as reflective of Reagan's views, was written and made public at the direction of White House chief of staff James A Baker III, who worked with national security affairs adviser Richard V. Allen after a National Security Council meeting at the White House.

Haig, who attended the meeting, issued the last strong warning to Moscow about Poland two weeks ago. But the administration has been on a seesaw about Soviet intentions.

Four days after Haig warned that the consequences of any Soviet intervention would be "grave and lasting," a State Department spokesman downgraded U.S. concern because of information that the Soviet maneuvers would involve fewer than 25,000 troops.

The new escalation of U.S. alarm was prompted by the duration, not size of the maneuvers. Although officials put the number of troops involved at about 10,000, the maneuvers had been expected to wind down soon. now they have been extended, perhaps indefinitely.

Reagan was asked as he left a reception for young Republicans in the East Room how serious was the situation in Poland. "Very serious," he answered.

"The seriousness of the situation" prompted the White house statement, he added.

Reagan said "certain movements, meetings and so forth" had raised U.S. concern but, asked if he meant Soviet troop movements, replied "not precisely that."

The statement said:

"The United States has watched with growing concern indications that Polish authorities may be preparing to use force to deal with continuing differences in that country between the authorities and labor unions. We are similarly concerned that the Soviet Union may intend to undertake repressive action in Poland."

It added: "We would like to make clear to all concerned our view that any external intervention in Poland, or any measures aimed at suppressing the Polish people, would necessarily cause deep concern to all those interested in the peaceful development of Poland and could have a grave effect on the whole course of East-West relations."

It pledged the United States to continued economic assistance of Poland as long as Polish authorities pursue a peaceful solution to their problems through negotiations. "It is in this spirit that we shall receive Deputy Prime Minister [Mieczslaw] Jagieiski in Washington next week," it said.