WASHINGTON (AP) _ Opponents of bank deregulation suffered several key defeats Wednesday as the House Banking Committee worked on long-awaited legislation hailed as the most important change in banking law since the Depression.

As originally written by the committee chairman, Rep. Fernand J. St Germain, D-R.I., the far-reaching bill granted bank holding companies some new securities powers, but would have prohibited them from underwriting corporate stocks and mutual funds.

But by a 35-14 vote, the committee approved an amendment by Rep. Doug Barnard Jr., D-Ga., to permit mutual fund underwriting, and by a 27-23 vote it endorsed an amendment by Rep. Thomas Carper, D-Del., to allow underwriting of bonds which are later convertible to stock.

Other securities activities permitted by the bill include the underwriting of corporate bonds, commercial paper, municipal revenue bonds and securities backed by mortgages and other consumer debt such as auto loans.

The committee defeated, 30-20, an amendment that would have struck corporate bond underwriting from the list of permitted activities.

The committee also adopted, 26-24, an amendment by Rep. Stephen L. Neal, D- N.C., watering down a provision designed to prevent the biggest banks from merging with the biggest securities firms.

The bill as originally written banned mergers between the top 35 firms in each category. The amendment applies the restriction only to the 15 largest of each.

However, in two other actions, the committee pleased members leery of new banks powers.

It adopted, 29-21, an amendment by Rep. Jim Leach, R-Iowa, which forbids banks from exercising the new powers until they meet international risk-based capital requirements adopted last year by Western nations meeting in Basil, Switzerland.

That would have the effect of requiring some of the nation's largest money- center banks, with heavy loads of shaky Third World debt, to sell new stock before entering new securities areas.

In the second action, the committee, 26-25, defeated an effort by Rep. Barney Frank, D-Mass., to weaken a section which forbids banks and affiliated securities firms from sharing the same name, logo, advertising or premises.

Rep. Frank Annunzio, D-Ill., the sponsor of the corporate bond amendment, argued that bond underwriting, particularly of high-yield, unrated securities known as ''junk bonds,'' is too risky for bank affiliates.

He said banks, failing at the highest rate since the Depression, have done a poor job of handling the deregulation of the industry that began in 1980 and should not be rewarded with additional powers.

''With deregulation, banks have suffered more failures than a guy in a singles bar with bad breath,'' he said.

Annunzio was backed by the senior Republican on the committee, Rep. Chalmers P. Wylie of Ohio, who said Congress should grant banks only a few new powers and wait to see how they handle them before proceeding.

But Rep. Charles E. Schumer, D-N.Y., another opponent of expanded powers, said he could ''read the handwriting on the wall'' and would confine his efforts to maintaining strong ''firewalls'' to insulate a bank's insured deposits from its securities activity.

''If we can keep that structure, we can at least look at ourselves in the mirror in the morning,'' he said.

Banks long have sought securities powers, which they have been barred from since the Glass-Steagall Act was passed in 1933 in response to scandals arising from the 1929 stock market crash and a subsequent wave of bank failures.

But banking groups say the St Germain bill, as written, would impose so many new burdens that they would rather see Congress fail to produce legislation and leave deregulation to the piecemeal process of regulatory rulemaking and judicial interpretation.

The bill also contains provisions ranging from new restrictions on banks' real estate and insurance powers to requirements that banks serve poor people by cashing government checks for non-customers and offering low-cost ''lifeline'' checking accounts.

Wylie said he would push to delete the consumer provisions, arguing that they could wind up hurting consumers by affecting the safety and soundness of the banking system.

Any bill passed by the House Banking Committee would have to be reviewed by the Energy and Commerce Committee before it goes before the full House.

Then differences between a House-passed bill would have to be reconciled with a bill passed overwhelmingly by the Senate four months ago.

That bill, which is favored by the industry, leaves open the possibility of banks underwriting corporate stock, requiring another vote on the issue by Congress by 1991.