In the grimmest assessment to date of the troubled savings and loan industry by a government official, Federal Home Loan Bank Board Chairman Richard Pratt acknowledged yesterday that one-third of the nation's 4,700 S&Ls -- with assets of $200 billion -- are "not viable under today's conditions" of high, volatille interest rates.

In Capitol Hill testimony, Pratt confirmed reluctantly that he gave these figures to a closed housing policy meeting last week. The figures he cited then and confirmed yesterday point to deeper industry trouble than federal financial regulators have hitherto acknowledged.

If nothing happens to help the industry and interest rates continue at the near-record levels of the last eight months, Pratt predicted to the commission that one savings institution every day will be reduced to a zero net worth, the point at which all financial reserves set aside to cover losses are used up.

He said that under a "downside but not widly, radically pessimistic estimate," the failue of these S&Ls could produce a $60 billion loss. The sale of assets and federal insurance would offset that figure by $15 billion, leaving a net loss of $45 billion, Pratt said.

Pratt previously had used more conservative figures in public, as he did again yesterday in prepared testimony to the House Banking Committee. He said that 263 federally insured S&Ls are on the regulatory agency's list of most-troubled institutions and that the $6 billion available in federal insurance would be adequate to take care of any losses caused by the failures or forced mergers of such associations.

When committee Chairman Fernand St Germain (D-R.I.) asked Pratt to confirm a more pessimistic analysis made last week to a meeting of the President's Commission on Housing, Pratt said an account in Washington Financial Reports, a Washington-based newsletter, was accurate. But he did not discuss it further.

A spokesman for Pratt said yesterday the bank board chief was painting a worst-case scenario to impress the housing commission of the seriousness of the situation, but that mergers and other rescue actions could prevent such a $60 billion loss from ever actually occurring. He denied that Pratt was depicting a worse situation in private than he admitted in public.

Although Pratt did not give a time reference in his talk to the commission, one economist who asked not to be identified said yesterday that he thought Pratt meant that if interest rates do not abate and savings and loans are not given any help, the potential loss within a year could be 10 times worse than the potential losses acknowledged at the moment.

Pratt also told the commission that at the end of April, the bottom 10 percent of the industry (395 associations) had a net-worth-to-assets ratio of 1.68 percent, whereas the industry average was 5 percent. Those S&Ls are losing some $3.50 on every $100 of assets.

Overall, the savings and loan industry still has $31 billion of net worth at this time. Moreover, deposits of up to $100,000 apiece are covered by insurance at federally insured institutions.

Pratt, accompanied by two other federal regulators, testified yesterday on ways to alleviate the situation. They pressed for passage of the socalled regulators' bill that was nixed a few weeks ago by the Reagan administration as being too costly. The current version calls for interstate and interindustry mergers as a way of assisting failing financial institutions. aThe bill also would permit cash infusions to troubled institutions as an alternative to liquidating them or merging them out of existence. CAPTION: Picture, RICHARD PRATT . . . sees possible loss of $45 billion