Del. Carrington Williams (D-Fairfax), a five-term member of the Virginia House of Delegates, announced yesterday that he will not seek re-election to the General Assembly this year because of his "firm intention" to run for the U.S. Senate next year.

But Williams, a Fairfax tax lawyer, stressed that "this is not a declaration of candidacy . . . it is a statement of intent" to seek the Democratic nomination for the seat being vacated by Republican William L. Scott.

Williams, 57, said he could not "as a practical matter and a matter of conscience" run for another two-year term in the House and at the same time be making plans for a statewide race. In 1971, Williams was runnerup for the party's nomination for Heutenant governor.

His announcement makes him the first of several Democratic state legislators to make formal their interest in seeking the Scott seat. Sens. Clive L. DuVal of Fairfax and James T. Edmunds of Lunenburg and Del. Donald G. Pendleton of Amherst also have been mentioned as possible candidates.

Williams said he believed it would be "premature" for him to announce his candidacy for the Senate until after he had traveled around the state, discussing prospects for his candidacy with party officials.

However, in his announcement and in an interview earlier this week, Williams appeared to be all but certain he would enter the race.