President Carter, continuing the spending crackdown the administration disclosed last week, has formally notified Congress that he intends to veto any fiscal 1979 appropriations bills that significantly exceed his own budgetary proposals.

In separate letters to the chairmen of the House and Senate appropriations committees, Carter warned bluntly he will follow congressional spending actions closely and "will not hesitate to veto any . . . which I do not believe the country can affort."

He also expressed chagrin about several bills now in the two committees which he said "contain significant increases to discretionary programs" - that is, those in which spending is controlled directly by Congress, rather than based on the number of taxpayers who are eligible.

Although Carter did not specify which measures he thought were excessive, sources indicated the President was referring to the pending Labor-Health, Education and Welfare, defense and public works appropriations bills. He urged the lawmakers to cut back the size of the measures.

The letter by the President marked the toughest language he has used so far in admonishing Congress to hold the line on federal spending levels. Carter has been relatively passive on budget issues in previous months. He has yet to veto his first spending bill.

The move appeared to be in line with a new tighter fiscal policy the administration has adopted. Carter announced May 13 he awas trimming the size of his tax cut by $5 billion, and last week he ordered a spending crackdown for fiscal 1980, for which planning has just began.

This week, the U.S. told its major trading partners in the Organization for Economic Cooperation and Development it would not join in a move by industrial nations to stimulate their economies more rapidly, as many European governments now want.

The move is being touted as part of a new anti-inflation effort. The administration is trying to reduce the size of the burgeoning federal budget deficit, in hopes of persuading the Federal Reserve Board to slow its rise in interest rates.

The letter Carter sent yesterday was drafted at the urging of James T. McIntyre, directof the the Office of management and Budget.