Federal banking regulators yesterday removed all interest-rate ceilings and minimum-deposit restrictions on savings accounts with a term of more than 31 days.

The change, which affects such popular accounts as the six-month money market certificate of deposit and the 18-month IRA/Keogh accounts, will take effect Oct. 1 at all banks, savings and loan associations and mutual savings banks. The regulators already had removed deposit-size and interest-rate restrictions on accounts with terms longer than 2 1/2 years.

Yesterday's decision will permit banks and thrift institutions to pay any level of interest they choose on any size account, as long as the deposit remains at the bank for 32 days or more.

The regulators also voted to ease mandatory penalties imposed on savers if they withdraw their funds before the specified term of the deposit. For deposits with terms of 31 days to one year, a depositor would forfeit one month's interest, while for accounts with terms of more than one year, the penalty will be three months' interest.

Now, depositors must forfeit three months' interest for early withdrawal of short-term deposits and six months' interest on longer-term deposits.

The only significant bank accounts that will have federally imposed interest-rate ceilings or minimum deposit requirements after Oct. 1 are passbook savings accounts, money market deposit accounts, checking accounts and NOW accounts--which are essentially checking accounts that pay interest.

Banks and thrift institutions can pay whatever interest rate they choose on the money market deposit accounts--which were introduced last December--but the depositor must make an initial deposit of at least $2,500. If the balance drops below $2,500, the depositor can earn an interest rate of only 5 1/4 percent.

NOW accounts have an interest ceiling of 5 1/4 percent. Passbook savings accounts--the traditional consumer savings account until the last few years--have a 5 1/4 percent ceiling at banks and 5 1/2 percent at savings and loans and savings banks. Old-fashioned checking accounts pay no interest.

The six top bank regulators that make up the Depository Institutions Deregulation Committee deferred a decision on whether to permit corporations the right to set up NOW accounts or to permit banks to pay interest on traditional checking accounts.

Treasury Secretary Donald T. Regan said that while he favors eliminating the prohibition of interest on checking accounts, he agreed with Federal Reserve Board Chairman Paul A. Volcker and others that the committee's legal authority to take such a step is "cloudy." Instead, the six-member committee, which is chaired by Regan, voted to ask Congress to pass a law repealing the prohibition of interest on checking accounts.

Bankers generally applauded yesterday's decision by the committee--which Congress set up several years ago to phase out federally mandated restrictions on accounts at banks and thrift institutions by 1986.

Citibank Vice President Robert Boyd said yesterday's action by DIDC "will make us more competitive across the spectrum." By approving money market deposit accounts, the regulators effectively deregulated short-term deposits. They also had deregulated deposits with maturities of two-and-a-half years or more.

But banks "faced a huge block of ice in the middle. The DIDC action melts that block of ice," Boyd said.

Banks had complained that they could not attract many depositors in the middle range of 32 days to 2 1/2 years and said that they were restricted in their ability to "match up" deposit maturities and loan maturities.

One of the reasons savings and loan associations traditionally face problems is that they take in very short-term deposits and use those deposits to make loans as long as 30 years. Bankers feel more comfortable if they have longer-term deposits behind longer-term loans.

Besides Regan and Volcker, the other members of the Depository Institutions Deregulation Committee are Federal Deposit Insurance Corp. Chairman William M. Isaac, Comptroller of the Currency C. Todd Conover, Federal Home Loan Bank Board Chairman Edwin J. Gray and National Credit Union Administration Chairman Edgar Callahan.