The Reagan administration has decided to permit Iraq, which has been suffering steady losses in its protracted war with Iran, to purchase six to 12 U.S.-made L100 transport planes on the understanding that they will be used for civilian purposes and not diverted to military use.

The move follows a Feb. 26 administration decision to remove Iraq from the list of countries formally regarded as supporters of terrorism. The decision on the planes is expected to encounter strong opposition from congressional opponents who charge that Iraq's record of terrorist activities against Israel and other Arab regimes and its increasingly desperate military situation make meaningless its promises not to misuse the planes.

Although Commerce Department officials yesterday refused to comment on the matter, the department has informed various congressional committees that it plans to issue a license on April 30 that will enable the Lockheed Corp., manufacturer of the L100, to sell six to the Iraqis. Congressional sources said the license will permit later revision of the deal to increase the sale by as many as six additional planes.

The 1979 Export Administration Act requires that Congress be notified when sales of equipment with a potential military use are proposed for countries on the terrorism list. Since Iraq now has been removed from the list, the ability of Congress to block the sale is very circumscribed, but some congressional sources said yesterday there may be attempts to attach a provision to the pending foreign aid bill that would bar the deal.

The L100 is the civilian version of the C130 cargo plane, and the Commerce Department has argued that limiting its sale to various Mideast civilian airlines for passenger use has put American manufacturers such as Lockheed at a disadvantage in competing for sales against European aircraft manufacturers.

Administration officials have acknowledged that pressure from the U.S. aircraft industry was the principal factor in the decision to open the way for sales of commercial planes to Iraq and, in more limited fashion, to Syria and South Yemen, two other Arab countries on the terrorism list.

The United States has declared a policy of official neutrality in the Iran-Iraq war, which has been in progress since 1980 and which lately appears to be tipping decisively in Iran's favor.

Despite fears in the Arab world that an Iraqi loss could cause the fall of President Saddam Hussein's radical regime and lead to political instability throughout the Persian Gulf region, U.S. officials have insisted there are no plans to give Iraq direct or indirect military aid.

The State Department acquiesced in the decision to take Iraq off the terrorism list on the ground that the regime there has decreased its support of international terrorist groups and should be encouraged toward further moderation.

Similarly, the Commerce Department has insisted that the proposed L100 sales are being approved solely on their merits as a commercial transaction and do not constitute a breach of the U.S. neutrality policy.

However, the easing of restrictions against Iraq has drawn strong protests from Israel, which Iraq regards as a sworn enemy.

Last month, four senators--Rudy Boschwitz (R-Minn.), Paul E. Tsongas (D-Mass.), Edward M. Kennedy (D-Mass.) and Larry Pressler (R-S.D.)--introduced a resolution charging that planes such as the L100 have a potential military use, that the administration has not substantiated its contention of an improvement in Iraq's record on terrorism and that the Hussein regime should be put back on the terrorism list.