The United States and its West European military allies are expected to agree today to establish an arms control group within NATO. The move represents a cautious first step by the Carter administration to share decision-making powers with the Europeans on the next round of U.S.-Soviet nuclear arms negotiations, diplomatic sources report.

The administration will endorse a West German proposal for an arms control planning group of today's meeting in Brussels of the North Atlantic Council, the treaty organization's political arm, according to the sources.

The most immediate impact of the move will be to provide some political breathing space for West German Chancellor Helmut Schmidt. He faces opposition at home from both left and right over the stationing of a new medium-range nuclear missile and possibly neutron artillery in West Germany once the Carter administration makes final production decisions.

The arms control group to be established today will not prsent its recommendations on a NATO approach to arms negotiations until the end of the year. Schmidt has already sought to still domestic debate and to avoid the kind of diplomatic impasse with the Carter administration that the 1977 dispute over the neutron bomb brought by saying that the new missile question had to be decided within NATO. However, his proposal for an alliance arms control group has not been made public before.

Schmidt appeared to try to diffuse his domestic criticism last month when he invoked NATO as the final voice on theater nuclear forces in Europe. He declared that Bonn would go along with a NATO decision to modernize the nuclear arsenal stationed in Western Europe if it were a joint devision, and if West Germany were not the only country on whose soil the new nuclear-tipped missiles would be based.

The proposal is expected to be endorsed by all council members with the possible exception of France, which has not yet made its position known. Diplomats predict that President Valery Giscard d'Estaing's government will remain neutral despite some concern in Paris about the impact the new planning group could have on France's proposal for a European Disarmament Conference.

France, which withdrew from NATO's integrated military command in 1966 but which has continued to participate in NATO at a political level, has also expressed reservations about the position of its nuclear forces in the next round of U.S.-Soviet strategic arms limitation treaty talks. These talks, known as SALT III, will cover nuclear forces stationed in Europe.

The treaty that is expected to emerge shortly from the SALT II negotiations will contain guidelines for the next round of nuclear bargaining, which for the first time will directly involve proposed cuts in U.S. systems based in-and Soviet weapons targeted on-Western Europe.

The West German proposal reportedly is deliberately vague and does and not tie the new planning group to SALT III or any other set of negotiations. But Giscard appears to be reluctant to be drawn into any arms control arrangement suggesting a departure from Charles de Gaulle's insistence on complete freedom of action for France in maintaining its own independent nuclear force. Giscard has held to a firm Gaullist position on his nation's nuclear force. CAPTION: Picture, CHANCELLOR HELMUT SCHMIDT . . . sponsor of proposal