Starting this week, a new federal law will put limits on the number of days that banks and other financial instutions can place holds on checks that customers deposit in their accounts.

The law has been hailed by consumer groups, who charged that Americans were losing millions of dollars annually because banks were freezing funds for undue amounts of time.Banks, however, complain that the new guidelines are confusing, proving costly to implement and will subject them to greater risks of fraud.

The controversy is being stirred up by the Expedited Funds Availability Act, which goes into effect Sept. 1.

Under the law, banks, savings and loans and credit unions must give customers access to deposited funds within one, three or seven days, depending on the type of check.

While many banks have been operating voluntarily under such deadlines, consumer groups argued that a law was needed because too many banks were freezing funds for periods as long as two weeks even though 99 percent of the time they were getting credit for the money within two days.

The U.S. Public Interest Research Group, which led the lobbying effort for the new law, charged that banks were making $290 million annually in interest on the money they were withholding from customers during the so-called "float period."

Additionally, the consumer research group said that banks were collecting another $145 million a year in bounced-check fees because funds were not released faster for depositors' use.

In broad terms, the new law requires that all local checks must be cleared by the bank within three business days. A local check is one written on an institution in the same metropolitan area or within the same Federal Reserve check processing region. There are 48 such regions in the country.

Non-local checks must be cleared within seven business days.

Funds deposited in the form of cashiers' checks, certified checks and government checks must be available to depositors the next business day.

By Sept. 1, 1990, the maximum hold period will drop to two days for local checks and five days for non-local checks.

Many bankers grumble that the new law will open the door to con artists. They complain that they will not have enough time to make sure deposited checks don't bounce or were not forged, especially in the case of the one-day deadline for government and cashiers' checks.

The regulations implementing the new law have proven to be complex, requiring a 600-page guide from the Federal Reserve, dealing not only with the deadlines but such matters as how checks should be endorsed and what information banks must supply to depositors.

By Oct. 31, all banks are required to have placed notices explaining the new rules in customers' monthly statements.