The redistricting plan drawn up by the Virginia House of Delegates was ruled unconstitutional today by a three-judge federal panel which nevertheless said the state may hold elections under the plan this fall.

The judges, in the first federal court decision in the nation on redrawing electoral lines following the 1980 census, ordered the General Assembly to produce a new plan by Feb. 1 to conform to the Supreme Court's "one-man, one-vote" guideline.

But the court refused to impose its plan or those suggested by several plaintiffs who had challenged the legislature's proposal in eight lawsuits. The judges said any court-ordered plan would mean a delay in the fall elections, scheduled Nov. 3, and lead to significantly decreased voter turnout.

Instead, the court declared that delegates elected this fall, who normally serve two years, will serve only a one-year term and ordered a new election in November 1982, under a revised plan.

Several plaintiffs said they were pleased the court threw out the legislative plan as violating the Constitution's equal representation principle, but said they would consider appealing to the Supreme Court the decision to allow this fall's elections.

"We're very concerned that if the elections are allowed to go forward under an unconstitutional plan, it might establish a bad precedent," said Frank Parker, attorney for the Washington-based Lawyers Committee for Civil Rights Under Law, a nonprofit legal group that, along with the American Civil Liberties Union, brought one of the principal lawsuits.

"Allowing an unconstitutional legislature to redraw the lines just perpetuates the problem."

"I'm concerned about the ramifications of having a legislature meet that is unconstitutionally selected," said Jane Reuss of Common Cause, who said her group would also consider an appeal.

The court's decision, unless overturned, would give Northern Virginia 21 delegates in the 100-member House, two more than it now has. Fairfax County would gain two new delegates and Loudoun County one under the plan, but Arlington and Alexandria would lose the "floater seat" delegate they shared through the 1970s.

The House in April passed a redistricting plan that protected the seats of most incumbents but even legislative leaders conceded would not meet constitutional standards.

The legislators had to return two weeks ago after the Justice Department rejected the plan on grounds it discriminated against blacks. By revising several district lines in Southside Virginia, the House came up with a two-member, mostly black district that satisfied the department.

But the redistricting plan's basic problem, as the court pointed out today, was the sharp deviations it established in population in electoral districts. The ideal district, using 1980 census data, would have 53,463 citizens. But the House plan allowed for districts with population as much as 14 percent below that standard and 12 percent above.

That created an overall disparity of about 26 percent between the state's largest and smallest districts, far beyond the 16 percent the Supreme Court in 1973 said "approach ed tolerable limits" in a previous Virginia case.

"The deviations among the populations of the districts . . . exceeds the limits tolerated" under the 14th Amendment's equal protection clause, the court said today. Noting previous rulings rejecting similar plans from Florida and Texas, the judges added that "the Supreme Court has not held a reapportionment statute with a deviation of this magnitude to be constitutional."

The court rejected another argument that the House plan diluted black voting strength by creating nearly a dozen multimember districts in urban areas.

But, writing for the panel, Judge John D. Butzner Jr. of the 4th Circuit Court of Appeals warned that if the plan to be submitted next year has multimember districts, it "must articulate precisely why a plan of single-member districts with minimal population variance cannot be adopted."

Other members of the panel were District Judges D. Dortch Warriner and Glen Williams.

Hampton Del. John Gray (D), chairman of the House elections committee that drew the plan, said he was not surprised by the ruling and said he expects his panel will meet later this year to begin drawing a new plan.

The decision "is better than what it could have been. At least it leaves it up to the legislature to redo the thing," Gray said. "I still think it was a good plan. I frankly don't agree with one-man, one-vote but that's what the law says."

Arlington Democrat Mary Marshall, also a committee member, said she expects the ruling will force single-member districts.

She said the lawmakers passed the previous plan out of "desperation . . . people kept hoping that somehow everything would come out all right. Now they know it won't."