The Senate Foreign Relations Committee yesterday approved a reservation to the SALT treaty declaring that the United States considers its proposed mobile MX missile system to be permitted by the arms control agreement with the Soviet Union.

The committee voted 11 to 1 for the reservation, which the White House says was not necessary. The only vote against came from Sen. George S. McGovern (D-S.D.), who opposes building the MX system.

By unanimous votes, the committee also approved reservations stipulating that:

The United States, in future negotiations, should seek increased access to information on strategic weapon deployment and development.

The terms of the treaty cannot be extended beyond the pact's 1985 expiration date without approval of two-thirds of the Senate.

Treaty protocol restrictions on the MX and cruise missiles are not a precedent for future negotiations.

In another development in otherwise slow day at the markup session, McGovern announced his intention to introduce an addition to the SALT treaty calling on the two nuclear superpowers to negotiate "significant and substantial reductions" in their strategic arsenals if they want favorable Senate action on future arms control agreements.

McGovern's proposal is scheduled to come before the full committee tomorrow.

The treaty yesterday picked up an enddorsement from a bipartisan group of six senators, all of whom had previously indicated their support. Sen. John Chafee (R-R.I.) principal spokesman for the group said Sen. Alan Cranston (D-Calif.), the majority whip and a key SALT supporter, had recently suggested that such a joint statement might improve the treaty's prospects.

Several of the senators predicted dire consequences if the Senate rejects SALT II. "I simply cannot imagine the disastrous result that would follow" rejection, Chafee said.

Others in the group were John Melcher (D-Mont.), who voted against the Panama Canal treaties last year; Gaylord Nelson (D-Wis.); Claiborne Pell (D-R.I.), Charles H. Percy (R-Ill.) and Robert T. Stafford (R-VT.).

In the Foreign Relations Committee, Sen. Jesse Helms (R.-N.C.), digressed from the business at hand to suggest that the committee's markup sessions were loaded in favor of SALT II, and that presidential counsel Lloyd N. Cutler might be an inadequate administration spokesman for the treaty.

Helms said that a treaty critic should be sitting at the witness tables in the hearing room where Cutler and other administration officials and members of the committee staff have been seated during the markup proceedings.

Paul S. Sarbanes (D-Md.) and Joseph R. Biden Jr. (D-Del) both sharply rebuked Helms for his comments. An aide to Helms said last night that the North Carolina conservative would "go after" Cutler again at today's committee session.