Starting April 1, financial institutions may offer any rate of interest they wish on all accounts. Also, penalties for early withdrawal of funds from certificates of deposit will be cut to a minimum by law, although probably not in fact.

This marks the final phase of the six-year process of deregulating interest rate ceilings mandated by Congress and administered by the Depository Institutions Deregulation Committee, headed by Federal Reserve Chairman Paul A. Volcker. The last interest rate ceiling -- 5 1/4 percent on passbook accounts -- will disappear at the end of the month. The only remaining prohibition is against the payment of interest on commercial checking accounts.

So will most statutory penalties for early withdrawal. Until now, banks and thrifts have been able to deduct up to three months' interest earned if a customer took money out of a certificate of deposit before it matured. The law has even permitted banks to retain part of the principal in cases where the money in a time deposit was withdrawn within 30 days of opening the account.

The maximum penalty will by law be cut back to one month's interest for early withdrawal of money from accounts with maturities of between seven days and 18 months. This applies only to accounts opened by institutions, not individuals. The single exception for personal accounts is a seven-day loss of interest if money put into a certificate of deposit is withdrawn within the first week. (This is necessary for legal reasons involving reserve requirements.)

Credit unions, which have not been required to levy premature withdrawal penalties, will have to abide by the same rule as of Jan. 1, 1987.

The penalties to go into effect in April are the maximum regulators are permitted to prescribe under the law that preceded DIDC. However, Fed governors this week gave strong indications they would try to get higher penalties under their mandate to preserve the safety and soundness of the banking system.