Transportation Secretary Drew Lewis has cautioned that Metro could lose $100 million in rail construction funds for 1983 unless Northern Virginia has a "stable and reliable" source of money by this summer to cover its share of Metro's operating subsidies.

In a Dec. 22 letter to Sen. John W. Warner (R-Va.), Lewis suggested strongly that Northern Virginia's special tax on gasoline, which will rise from 2 to 4 percent this summer, would cover too small a part of Northern Virginia's subsidy bills and therefore would not meet federal standards governing release of construction funds.

The gasoline tax approach was not specifically rejected in the letter and a spokesman for Lewis said yesterday that the secretary was not telling Virginia how to proceed. The letter, however, did say that Lewis would look for further assurances from the state and local governments that the subsidies could be paid.

Warner, also speaking through a spokesman, said he is assuming Lewis has decided that the 4 percent tax is inadequate and that further action is necessary.

After receiving Lewis' letter, Warner and other members of the Virginia congressional delegation wrote to Gov. John N. Dalton requesting his help "to assure that the commonwealth has done its part by completing the work to obtain a stable and reliable source of funding . . . ."

In fiscal 1983, Metro officials want to use the first $100 million of $1.7 billion in subway building funds authorized by 1979 legislation known as the Stark-Harris Bill.

The money is available only if Maryland, Virginia and the District have created what the federal government considers stable and reliable revenue sources by this summer to cover all or most of their growing subsidy bills. Congress was concerned that the local jurisdictions' general funds, which have covered most of the bills, would break under the pressure of subsidies and debt service. Local bills are expected to total more than $200 million in fiscal 1983.

The Maryland state government now covers about 75 percent of Montgomery and Prince George's counties' bills, while the D.C. City Council appears set to pass a bill whereby certain fees and taxes would be put into a special fund designed to give 100 percent coverage.

Though Lewis did raise some questions over Maryland's approach, federal doubts center on Virginia's attempt to comply: the gasoline tax. Lewis estimated the tax would pay only 24 to 42 percent of Northern Virginia's shares of operating aid and debt service. A 24 percent level, he said would be considered "not acceptable."

The Virginia legislature two years ago approved the tax, which is paid only in Fairfax, Arlington, Alexandria, Falls Church and Fairfax City, with the assurance of Carter administration officials that it would meet the vaguely defined "stable and reliable" standards. However, the Reagan administration began reviewing it a year ago and many transit officials feel it has drawn different conclusions.

"It's quite disturbing. I'm looking into it right now," state Sen. Adelard L. Brault (D-Fairfax), head of the Northern Virginia delegation, said of Lewis' letter. The delegation will meet this Wednesday to discuss the problem before this year's legislative session convenes, he said. Proposals for further state action could meet serious opposition from downstate legislators.

Uncertainties over Stark-Harris funding come as the other big component of Metrorail's fiscal 1983 capital budget, federal interstate transfers, is also running into difficulties.

Metro officials had hoped for $275 million in transfers--in which proposed interstate highways are canceled and the money traded in for funds for transit or other projects--but now there is evidence that the administration is thinking in terms of only $195 million in transfers.

Moreover, D.C., where the highways were to be built, has the right to decide how the transfers are spent.

So far, D.C. has maintained that it will use most of them to resurface roads and repair bridges, giving the remainder to Metro. Federal officials, meanwhile, seeking to cap total spending, have refused to offset that loss by increasing the $100 million proposed for Stark-Harris funding, Metro officials say.