The Export-Import bank has decided to provide up to $1.5 billion in loan guarantees for U.S. exports to cash-starved Brazil, officials said yesterday.

Congress has a month to review the proposal before the Ex-Im bank can make the credit guarantees, officials said. It will likely be November before loans are actually arranged, but the Ex-Im board made the decision this week in order to give Congress enough time to review it before the end of the current fiscal year, an administration official said.

The Ex-Im board also decided to give Mexico up to $500 million of credit guarantees and insurance in support of U.S. exports, a spokesman said.

Brazil, the largest Third World debtor with almost $90 billion owed, has been hovering on the edge of bankruptcy for weeks. Its creditors have refused to advance more money until the nation reaches agreement with the International Monetary Fund on a new economic program.

Last week, the IMF and Brazil reached a tentative agreement on new economic measures. This was the signal for international banks to move ahead with negotiations on new loans to help Brazil meet its bills this year and next.

The U.S. government plans to join other governments in helping provide cash to Brazil through a package of increased export credit guarantees, the Reagan official said. The U.S. contribution to the Brazilian package will "certainly" amount to less than 50 percent of the total, the official said.

It will be some time before the negotiations are completed between lender governments on how much each will contribute to Brazil through their export credit agencies, he said. The IMF is not expected to give formal approval to the Brazilian economic measures until late October, sources say.

Also, Brazil yesterday sought to delay repayment of capital loans from commercial banks falling due in 1984, the Associated Press reported. Brazil hopes to transfer $5.1 billion of capital repayments due next year into new eight-year loans with a 2 1/2- to three-year grace period.