The nation's thrift institutions yesterday reported a net deposit loss of $6.6 billion in April, the biggest monthly savings outflow in their history.

Federal Home Loan Bank Board Chairman Richard T. Pratt, noting that the outflow showed consumers' preferences for money market mutual funds, declared that "negative savings flows at savings and loans can be expected to place further pressure on the availability and cost of home mortgages available to consumers."

Meanwhile, these are indications that increasing numbers of thrift institutions are preparing to become more competitive with money market funds through the introduction of new saver certificates of their own.

Their hopes for government restrictions on money funds now virtually dashed, thrifts are reassessing how to compete. While William B. O'Connell, executive vice president of the U.S. League of Savings Associations, is counting on Congress to pass legislation permitting tax-exempt savings certificates, others are planning to challenge the funds outright by offering market-rate repurchase agreements and certificates with market equivalent rates, both insured and uninsured.

Community Savings and Loan of Gaithersburg has just announced formation of what amounts to the first insured money market fund in this area. Called the Community Daily Money Account, it pays market rates (currently 16.67 percent) on deposits of $3,000 or more. The rate may fluctuate daily, and the funds may be withdrawns at any time without penalty. The principal and credited interest are insured to $100,000. Checks in $500 minimums may be written on the account.

Community President John D. Faulkner said, "We're out to get the money market funds." Several other savings and loans in the Washington area also reportedly are interested in setting up such accounts.

Customers withdrew $4.63 billion more from the country's 4,000 savings and loans last month than they deposited, and the net loss at mutual savings banks for April was $2 billion, resulting in the total $6.6 billion loss for the thrifts.

As large as the $4.63 billion figure is, it represents just 0.91 percent of S&Ls' total deposits. (Even after an interest credit of $1.8 billion, net deposits still fell by $2.8 billion, the first absolute negative return in years for one month.) Although April is traditionally a poor month for savings, because of tax payments, the situation was exacerbated by the higher interest rates paid by money funds and other investments. Customers did not renew $4.5 billion in certificates of deposit in April, contributing to the S&Ls' outflow.

Mortgage activity was relatively well-maintained during April. Loans totaled $5.4 billion, 10 percent more than during more than during the previous month and 20 percent more than during April 1980. Funds for the loans came largely from borrowings within the FHLBB system, according to its chief economist, Richard Marcis. Most of the loans represented commitments made in early March when the mortgage interest rate was 15.4 percent. As rates have risen, commitments have tapered off, and loan money will be tighter next month.

April actually was even worse for the 460 mutual savings banks, located primarily in the Northeast. The $2 billion net outflow amounted to 1.31 percent of total deposits. Since the beginning of this year, mutual savings banks have lost some $4.1 billion in deposits, following heavy losses the previous year.

Early indications are that May again will show a positive savings flow, thanks in part to the slight interest-rate advantage thrifts currently enjoy over money market funds.