A revival of longer-term charters for the tanker market is forecast as the number of laid-up vessels shrinks.

SS&Y Research Services Ltd., the research arm of the London company Simpson, Spence & Young Shipbrokers Ltd., points out in its annual shipping review that a backup of ships that can be reactivated in the event of an unexpected upturn in demand no longer exists.Consequently, "it would be extremely unwise to operate only in the spot market," the broker warns charterers.

British Petroleum Company PLC, the major U.K. oil company, recently concluded several charters for periods of up to 10 years, a decision that has won considerable praise from tanker owners and brokers.

"We welcome more medium and long-term charters," said Ronald Bergman, the president of the S wedish shipping line Nordstrom & Thulin AB. Mr. Bergman ideally would like a mixture of spot and longer-term charters in order to ensure greater stability and facilitate forward planning.

Lars Carlsson, president of Concordia Maritime AB, the Swedish tanker operator, also gave the BP deal a warm welcome.

Not every tanker owner prefers the security of longer-term contracts, however. Basil Papachristidis, chairman of Papachristidis Ltd. and the former chairman of the International Association of Independent Tanker owners, has consistently argued in favor of the spot market for all business and claims that medium or long-term charters distort the real supply and demand position.

The rates being paid by BP for four very-large crude carriers represent ''excellent to adequate returns to the owners," according to SS&Y Research, "but are a long way short of the rates required to cover the cost of vessels now being ordered."

The SS&Y report says more interest in period business and longer-term arrangements is already apparent, the outcome of the tighter supply situation.

The brokers also note that the world's tanker order book has grown by more than 50 percent since the middle of last year, with a "staggering" 34 vessels scheduled for delivery in 1992.

Thus, although demand for tankers this year and next is relatively healthy, the report warns of the dangers of a tonnage surplus reappearing as scrapping rates remain low and new ship construction accelerates.

"The two sides of the equation must move in harmony, otherwise we are in danger of having a real imbalance in the market," SS&Y research predicts.