Boeing Co. is expected to announce today that Singapore Airlines will purchase up to 10 of its latest-model 747 jumbo jets with a stretched upper deck, sources said yesterday.

The order, reportedly for eight planes with an option for two more, could be worth as much as $850 million. The price tag has been estimated at about $85 million for each plane by delivery time in two or three years.

The order for the new version of the 747 comes at a time when orders for the aircraft have been falling off and the assembly line in the vast Everett, Wash., plant has been slowing down. The order could stem further layoffs at the Boeing facility.

The announcement is expected to be made at a press conference today in New York by Boeing Chairman T.A. Wilson. According to sources, Wilson also is expected to announce that there will be a "competition" between Pratt & Whitney and Rolls-Royce to see which company's engine will be picked to power Singapore Airlines' new planes.

Pratt & Whitney engines are on most 747s; some original-equipment Rolls-Royce engines are said to be on a small number of 747s bought early on by airlines of the British Commonwealth. The Rolls engine has been used primarily on Lockheed Corp.'s L-1011, a plane whose production will be halted in 1984 after the last of the planes currently on order is delivered.

The plane Singapore Airlines will be buying is a new version of the 747, the largest of the available commercial aircraft, which entered commercial service in 1970. Without changing the wing or lengthening the fuselage of the standard model, the new version's upper deck has been extended 23 feet to the rear to accommodate up to 44 more passengers than the standard airplane. The new plane can seat a whopping 550 passengers.

Swissair was the first airline to order the new version of the 747 with an order for five of the planes; delivery is to begin in March 1983. South African Airways has placed an order for two of the planes.