The cost of a 2,769-mile pipeline to carry natural gas from Alaska to the lower 48 states has increased to dramatically that the consortium of firms backing it s construction may not be able to finance the venture.

Three major oil companies and the consortium have agreed the oil companies should own 30 percent of the Alaskan portion of the line. But that portion alone is now estimated to cost $30 billion compared with the $12 billion it was expected to cost a little more than a year ago.

In early 1980, the cost of the 2,769-mile system reaching from Alaska's North Slope through Canada with branches running to California and Illinois was pegged at $24 billion.

The enormous increase in the cost, plus the current availability of unused gas production capacity in the lower 48 states, will make it difficult for the pipeline group to borrow the $22.5 billion it will need to raise in addition to $7.5 billion worth of equity financing.

In addition, since equity participation by the oil companies in the pipeline is barred by law, the backers will have to persuade either President Reagan or Congress to lift that prohibition imposed in 1977. The group says terms of an executive order issued by President Carter that year, and incorporated into legislation, allow the president to do so without congressional action if he chooses.

Nevertheless, some key members of Congress, including Rep. John Dingle (D-Mich.), chairman of the House Commerce Committee, have strongly opposed participation in the pipeline by any of the North Slope gas producers. The three oil companies involved are Exxon Corp., Standard Oil Co. of Ohio and Atlantic Richfield Co.

Under terms of the agreement between the trio and the Alaska Northwest Natural Gas Transportation Co., a consortium of gas pipeline and distribution companies headed by John G. McMillian, the gas producers would own 30 percent of the pipeline. The consortium is supposed to put up $5.25 billion in equity capital and borrow $15.75 billion. The oil companies would provide $2.25 billion in equity and $6.75 billion worth of debt.

Representatives of the group briefed officials at the Bank of America and Citibank, the nation's two largest commercial banks, on the proposal last week. Some members of Congress were notified of the agreement by letter late in the week.

"A lot o people are going to be flabbergasted by this," one congressional source said. "The jump in price comes at a time when there is a glut of gas on the market. The private bankers are probably going to say there is no way we will go along with this."

A key problem could be that the proposal allows only $3 billion of the 30 billion for cost overruns. The bankers might insist on a federal guarantee of financing to cover any additional overruns to provide more safety for their loans, sources close to the project said. In the past, there has been considerable opposition in Congress to that possibility.

However, during his official visit to Canada earlier this year, President Reagan gave firm assurances to Prime Minister Pierre Elliott Trudeau that the American portion of the pipeline would be constructed. The eastern and western legs of the system in the lower 48 states are under construction, as is the Canadian portion that would link them to gas fields in Alberta. Canadian gas will be transported through both legs until the remainder of the system is completed, expected in 1986.

The letter to several members on Capitol Hill said that the necessary changes in the 1977 executive order must be made before Oct. 23 when this session of Congress is scheduled to end, or else the construction timetable would slip, further increasing the projected cost.

The $30 billion figure is in terms of "as spent" dollars; that is, it includes inflation expected between now and 1986. It also includes $6 billion for the plant that would be needed to process the gas to bring it up to so-called pipeline quality, and the cost of servicing the huge debt that would be incurred at an assumed 14 percent interest rate.

Gas reserves at Prudhoe Bay in Alaska are estimated at 26 trillion cubic feet, with additional gas likely to be found not far away as a result of offshore dilling under way. The completed pipeline system would carry about 2.4 billion cubic feet a day, about 5 percent of U.S. consumption. Later, gas from the Canadian Arctic might also be moved through the system south of the Yukon.

Customers for the Alaskan gas would have to pay a price high enough to cover the cost of its transportation, including a return to the investors building the pipeline. If the Reagan administration proceeds with its plans to speed deregulation of natural gas prices, there might not be sufficient low-cost gas being sold with which the high delivered cost of the Alaskan gas could be averaged in order to make it attractive to customers, the sources said.