The board of directors of the trouble-plagued Diplomat National Bank last week turned down a $2 million stock bid that was twice as high as any other in order to guarantee firm control of the bank by Koreans, sources said yesterday.

Instead, the directors accepted a $1 million bid for controlling interest of the bank from a partnership composed of South Korea's largest bank and 11 Korean trading companies.

Korean ownership of Diplomat has been a controversial issue ever since revelations that agents of the South Korean government secretly had bought control of the institution shortly after it was chartered in 1975 to help serve the Asian-American community.

Diplomat's board of directors has been told by federal bank examiners that unless it raises at least $1 million in new capital it will be dissolved or forced to merge with a stronger bank.

D.C. financier Leo M. Bernstein, who last month sold off his 15 percent interest in National Savings and Trust Co., where he is vice chairman, offered $2 million for the 500,000 controlling shares of Diplomat stock put out for bid earlier this month. The Cho Heung Bank of Korea and its partners bid $1 million.

However, a complex agreement between Bernstein, the Korean partnership and the bank made it highly likely, sources said, that control will fall to Bernstein.

The U.S. Comptroller of the Currency must approve the decision by Diplomat's board to sell to the Korean interests. The three parties agreed that if that approval, which involves an elaborate regulatory review, is not given by 5 p.m. today, the shares will be sold to Bernstein.

Under federal law, the Comptroller of the Currency must be given up to 60 days to review and approve potential new owners of a national bank. The Korean bid was not given to the comptroller's office until last week.

One source familiar with the Korean partnership's position characterized its chances of meeting today's deadline and winning control of the bank as "very slim."

Additionally, some of the Korean members of the Diplomat's board of directors reportedly have complained that federal regulatory officials are making it virtually impossible for the directors to sell to the buyer of their choice, the Cho Heung partnership.

Sources said that approval of Bernstein as a new owner would be a relatively simple process because he is a known quality in the local business community. By contrast, a review of the Korean partnership would entail a detailed study of 12 large and complex corporations with complicated international operations.

A spokesman for the comptroller's office would not comment yesterday.

Diplomat's president, John Ricche, said that his board of directors had passed a resolution to keep all of the negotiations over the change in ownership secret. "It sounds like you've got some pretty good information there, but I'm not in a position to confirm or deny it."

Ricche did say that the announcement of who the bank's new owner will be is scheduled for 5 p.m. today. "We honestly don't know who the bank's changing hands with at this point," he said, "but at 5 o'clock we will have a new owner."

Bernstein also could not be reached for comment.

Raymond S. Sczudlo, a Washington lawyer for Cho Heung Bank, said he was retained last Friday by the South Korean bank to help it and its partners win regulatory approval as new owners of the bank.

He said the 11 trading companies that form the partnership with Cho Heung are substantial merchant firms.

Diplomat's financial troubles are well known in Washington banking circles. Ever since it opened its doors 4 years ago, it has failed to post a profit. Its 1979 losses were $1.1 million on deposits of a little more than $8 million.

One source familiar with the bank's current condition said that its working capital funds have slipped to under $200,000 in recent months.

That prompted the comptroller to issue a "cease and desist" order, sources said, ordering Diplomat's directors to find new sources of capital by July 15 or face closing the bank's doors. That deadline was later extended until today.

Early this month, bank officials announced that they had received 10 firm offers to buy at least $1 million worth of new Diplomat stock. Bank officials would not disclose the identity of the bidders.

Though the bank is relatively small by even Washington's modest standards, the potential to revive the bank and turn it into a powerful, profitable and influential force in the community gives the ownership competition a high stakes aura.

Banking industry sources say that it is virtually impossible to get a new charter for a national bank in D.C. and, therefore, the limited number of charters are extremely valuable commodities, much like television station licenses.

Diplomat barely had been chartered for barely six months when, as a result of congressional investigations into South Korea influence buying in the United States, it was discovered that Korean evangelist Sun Myung Moon and his followers had secretly acquired nearly half of Diplomats stock.

Moreover, Tongsun Park, the central figure in the Korean influence-buying scandals, was discovered to own 10 percent of the bank's stock through three front men who held his interest in their names. Federal investigators even found that a Korean-born Washington businessman had used $50,000 in funds from the Korean Central Intelligence Agency to buy stock in the bank.

A year ago this month, Moon's Unification Church signed a consent decree with the U.S. Securities and Exchange Commission promising not to violate U.S. securities laws in the future while admitting no wrongdoing in the past. CAPTION: Picture, LEO M. BERNSTEIN . . . his $2 million stock bid rejected