Charles Robb's recent op-ed piece, "Support the Peace Plan and the Contras" {Jan. 25}, proposes a paradoxical, and therefore impossible, policy. Moreover, the arguments used by Mr. Robb reveal a blatant disregard for the content and context of the statements of the Central American leaders themselves on this subject.

New and continued contra aid and peace in Central America are incompatible. All five Central American presidents have recognized this fact, and all five have publicly called upon the United States to end its illegal funding of the contras. The author of the peace plan, President Oscar Arias of Costa Rica, has emphatically urged the United States to cease and desist this misguided policy.

When the International Verification and Follow-Up Commission, created in accordance with the Guatemalan Procedure, met in Panama on the eve of the San Jose' summit, the governments of all five Central American countries joined the nations of the Contadora and Support Group, the United Nations and the Organization of American States and condemned contra aid, saying:

"In spite of the exhortation of the Central American presidents, the government of the United States of America maintains its policy and practice of providing assistance, military in particular, to the irregular forces operating against the government of Nicaragua. The definitive cessation of this assistance continues to be an indispensable requirement for the success of the peace efforts and of this Procedure as a whole."

On Jan. 16 in San Jose', the Central American presidents reached an agreement on how to deal with the problems of Central America. They agreed against further funding of the contras. The Reagan administration can heed the counsel of its allies in the region and cooperate with the Central American peace process or it can work to undermine the progress already attained and dash prospects for peace by pressing ahead with more contra aid.

A choice does exist. The administration can either support the Central American peace efforts and end U.S. funding of the contras or it can continue funding of the contras and effectively kill the Central American peace process. It might be worth reminding Mr. Robb and the administration, that like all things in life, you just can't have it both ways.

CARLOS TUNNERMANN

Nicaraguan Ambassador to the United States

Washington