The creditor left holding the bigest bag in the bankruptcy of Britain's Laker Airways is none other than Uncle Sam, in the form of the Export-Import Bank.

Laker owes the bank $86 million on an 8 3/8 percent, 10-year loan approved just two years ago. In addition, the Ex-Im Bank guaranteed repayment of $61 million that Laker borrowed from Export Funding Corp., a private consortium in New York.

That brings the Export-Import Bank's total exposure to $147 million, which bank officials said yesterday is secured only by a first mortgage on five of Laker's DC10-30 wide-body aircraft.

Those planes represent an asset of doubtful value in the current depressed airline market, industry sources said yesterday. A recent worldwide survey found 102 wide-body aircraft already being offered for sale or long-term lease by current users, according to authoritative industry sources.

The Export-Import Bank provides loans on favorable terms to foreign buyers of U.S.-made products. A bank spokesman said the Laker loan probably would not be made if it came up today, partly because the Reagan administration has cut available funds and partly because U.S. firms have a near-monopoly on the market for long-range aircraft and do not need assistance in selling abroad.

McDonnell Douglas Corp. of St. Louis, which builds the DC10, said it faces a maximum possible loss of $50 million in the Laker bankruptcy because it participated in an international group that lent additional money to the airline. The company said it anticipated the loss, however, and already had accounted for it in its balance sheet.

"When we announced our fourth-quarter earnings last month, we said our earnings had been hurt by a $50 million pretax provision for probable restructuring or default on some commercial aircraft transactions," McDonnell Douglas spokesman Ray Defrey said. "That was for Laker."

McDonnell Douglas and General Electric Co., maker of the DC10's engines, hold a second mortgage on some of the planes in the Laker fleet.

General Electric, like McDonnell Douglas, said it would "incur a small loss, which has already been adequately covered by reserves" and noted in earnings reports. The company did not say how much its loss could be, but industry sources said it was about $10 million.

It probably will be some time before the full extent of the losses is tabulated because a receiver has been appointed in Britain to manage Laker's assets and come to terms with creditors. Industry officials said a rebound in the world economy could bring a sharp increase in the demand for commercial aircraft.

Because current demand is so slack, however, McDonnell Douglas and Fokker B.V. of the Netherlands terminated an agreement on joint design of a new 150-passenger plane, they announced yesterday.