Former Washington Star owner Joe L. Allbritton is negotiating to buy the largest block of shares in Riggs National Bank, the biggest financial institution in metropolitan Washington, informed sources said yesterday.

Allbritton, a self-made millionaire with extensive experience in bank management and ownership, reportedly has offered more than $30 million to buy the stock in Riggs now owned by Jorge E. Carnicero, an Argentine-born Northern Virginia businessman who bought Riggs shares in the 1970s and established himself as the bank's major owner at the end of 1978.

Banking and securities industry officials said Allbritton has offered $80 a share for Carnicero's holdings, a substantial premium above the stock's current market value. Riggs stock has been rising in recent days in unusually heavey trading and closed yesterday at $47.50 a share, a new high for the last 12 months.

Carnicero could not be contacted yesterday for comments about the reported negotiations, which representatives of the two men are conducting in Washington. Through a spokesman, Allbritton said he has not agreed to buy or not to buy Riggs stock "or any other . . . we are always looking."

One informed source said that if an agreement is reached it could be finalized Monday, if not earlier.

Any such transaction would require approval of federal banking regulators, and some sources said that one question regulators may raise concerns Allbritton's interests in Texas banks.

Carnicero, chairman of the Dynalectron Corp., a diversified technical services and energy company in McLean, was reported to be traveling in Argentina yesterday. But it is known that he has been approached previously about selling his Riggs interests.

After Carnicero's ownership became public knowledge early in 1979, he insisted in an interview with The Washington Post that he was interested in the stock as a business investment only.

Although Carnicero has not played a role in Riggs bank management, he has been a director since 1977. With 10 percent of Riggs shares in his possession by the end of 1978, he installed two persons on the bank's board of directors the next year to represent his interests. They were his daughter, Jacqueline Carnicero Duchange, and long-time business associate Merlon F. Richards, Dynalectron's chief operating officer.

Exact amounts of Carnicero's stock in Riggs could not be determined yesterday, but informed sources estimated his holdings of common stock at almost 15 percent of 2.99 million shares outstanding. He owned about 300,000 shares, or 10 percent, at the end of last year but has been buying shares since then. Argentine investors have a minority interest in Carnicero's Riggs bank holdings.

Daniel J. Callahan III, the president of Riggs, said last night that he was unaware of any unusual stock market activity in his bank's stock or any talks by representatives of Carnicero and Allbritton. "I really can't say anything. I really don't know," said Callahan.

Allbritton was in Houston yesterday, where he still has a home and extensive business interests. He also has a newly remodeled house in Washington, where he purchased The Star in 1974 from descendants of that newspaper's founding families. Four years later he sold the financially ailing publication to Time, Inc., but retained other Star communications properties and reportedly made a profit of some $70 million on what he has estimated was $65 million investment in The Star and its associated businesses.

Sources said the Washington investment banking firm of Folger Nolan Fleming Douglas, which worked with Allbritton in his takeover of The Star, had issued a "nonsolicitation" order to its staff regarding Riggs stock, which means that brokers cannot seek to buy or sell the bank's shares with customers, although they can take unsolicited orders.

Such an event normally takes place only when a major business deal is close to fruition and the sources yesterday emphasized that the negotiations could fail. Folger Chairman John C. Folger did not return a reporter's call about the reported talks and the nonsolicitation memo.

One source said buying a large block of Riggs would be consistent with Allbritton's career and business dealings. He joined the board of directors at American Security Bank after buying The Star, but stepped down from the board quietly in January 1979, after selling the newspaper to Time. Sources said he has been buying small amounts of Riggs stock since then.

Allbritton, 56, has a net worth estimated at between $100 million and $150 million, making him one of America's wealthiest persons.

A native of D'Lo, Miss., Allbritton moved with his family to Houston when he was 5 years old and by age 33, he had made his first $1 million.

He began with a law practice and, in 1957, joined some friends to establish a savings and loan association in Houston. By the late 1960s, he had acquired control of two Houston banks, which he merged into Houston Citizen's Bank & Trust Co., later acquired by a Dallas bank in a deal that made Allbritton the largest stockholder. He remains the largest single shareholder in the holding company formed at that time, First International Bancshares.

His other holds include a medium-sized insurance company, Pierce National Life Insurance, of Los Angeles, of which he is chairman; a chain of mortuaries in California, Pierce Brothers; a bank in Luxembourg; and a variety of other real estate and business investments. Virtually all of Allbritton's business activities are channeled through a holding company, Perpetual Corp., of which is chairman.

Since selling The Star in 1978 for approximately $28 million, Allbritton also has remained active in communications. He retained ownership of profitable WJLA-TV (whose initials are those of Allbritton's) here after selling The Star, which he blamed on Federal Communications Commission policies that prevented him from owning both the newspaper and television station in one market.

In addition, Allbritton Communications owns newspapers in Massachusetts and New Jersey and television stations in Lynchburg, Va., and Charleston, S.C. In the fall of 1979, Allbritton's firm borrowed $40 million for debt consolidation and other purposes by selling notes to institutional investors at an interest rate of 10 1/2 percent, with First National Bank of Chicago serving as Allbritton's financial adviser.