Amid mounting indications that Israel may be expelled tomorrow from the U.N. telecommunications agency, Secretary of State George P. Shultz warned yesterday that the United States will immediately cut off funding to the United Nations or any of its affiliated organizations that vote to exclude Israel.

Shultz said the United States views with "grave concern" political acts such as last month's rejection of Israel's credentials by the International Atomic Energy Agency and warned that more such moves would threaten to "do grave damage to the entire United Nations system."

The United States in the future will not just walk out of meetings, as it did last month when the atomic energy agency rejected Israel's credentials, but will immediately cut off funding until "Israel's right to participate is restored," Shultz said.

The secretary of state said the United States will suspend participation in and payments to the International Telecommunications Union if Israel is excluded this week and will cut off funds to the United Nations if Israel's credentials are rejected later this month by the U.N. General Assembly.

A State Department spokesman later emphasized, however, that while the United States would suspend participation in the General Assembly if the credentials vote Oct. 25 goes against Israel, the United States will "not withdraw from the Security Council or from other U.N. organizations where there is no wrongful action against a legitimate member."

Shultz also confirmed that the United States has decided to suspend paying the remaining $8.5 million of its 1982 dues to the atomic energy agency until the outcome of a reassessment now under way of U.S. participation in that organization.

Shultz issued the public warning yesterday after receiving a new cable from the U.S. delegation attending the International Telecommunications Union conference in Nairobi, reporting that the tide appeared to be shifting more heavily against Israel in that body.

The U.S. delegates, who earlier last week sent word that an Arab attempt to expel Israel from the 157-member telecommunications union seemed likely to carry by three or four votes, reported in a cable yesterday that their latest head count showed Israel losing by 12 votes, sources said.

The International Telecommunications Union, like the atomic energy organization, is a U.N. specialized agency.

While Shultz said the United States would suspend further payments to the telecommunications union if Israel were excluded, a State Department source said that the U.S. assessment to that agency for this year had been paid in full and that the 1983 assessment of $2.4 million is not due until Jan. 1.

However, the threat to suspend U.S. payments to the United Nations if Israel is excluded from the General Assembly could have a far more immediate impact. The United States owes the United Nations $149.4 million for the balance of calendar 1982, and the U.N. is "planning to receive that amount by early November," the source said.

That sum is used to fund operations of the U.N. Secretariat, the General Assembly, the Security Council, the Conference on Trade and Development, the Industrial Development Organization, the five regional economic commissions and the Human Rights Commission. The basic U.S. contribution does not, however, include support for budgets of the World Health Organization, the Food and Agricultural Organization, the International Civil Aviation Organization, the U.N. Development Program or U.N. peace-keeping forces. Sources said yesterday that it does not appear these would be immediately affected by any suspension of U.S. payments to the United Nations.

The resolution to oust Israel from the telecommunications union was introduced by Algeria and is supported by all of the Arab states except Egypt and Lebanon.

At the United Nations, the move to reject Israel's credentials for the current General Assembly session has been led by Iraq and Libya. However, Iran has privately informed delegations that even if Arab delegations back down, it is determined to challenge Israel's credentials when the question comes before the General Assembly Oct. 25, sources said.

Shultz, in his statement yesterday, said excluding Israel from the General Assembly would not only be contrary to the principles of the United Nations but also "a clear-cut violation of the U.N. Charter.

"We trust that the majority of nation members of the United Nations and all its agencies recognize the grave dangers of any further attacks on Israel's fight to participate in United Nations bodies and will work to turn aside such initiatives," he said.