Marriott Corp., in one of its strongest signals yet that tougher times are ahead, said yesterday it will put on hold almost all of its plans to build new hotels and other projects next year in an effort to cut construction costs by nearly one-third.

The cutbacks mark the second time in five months that the Bethesda-based company has reduced its spending plans amid a slowdown in the real estate and hotel markets nationwide.

Hotel construction is of critical importance to Marriott because the company derives a considerable proportion of its profit from building and selling hotels and motels to groups of investors. The company -- the largest in the Washington area -- also manages hotels and food-service operations.

The new budget reductions will delay the growth of three hotel lines on which Marriott had previously pinned its expansion hopes: the moderately priced Fairfield Inns and Courtyard by Marriott and the "extended-stay" Residence Inns chain.

About 20 to 30 projected hotels in each line, slated to be built around the country, will be delayed for a year to 18 months, said David Chichester, Marriott's assistant treasurer.

Investors worried about the company's prospects have helped to drive down the price of Marriott's shares by more than 50 percent over the past three months. Its stock closed yesterday at $11.50 a share, up 62 1/2 cents for the day.

Marriott said it plans to spend $650 million next year on various projects, mostly to finish hotels and retirement communities already under construction.

At the end of 1989, the company had said it would budget $1.2 billion for such projects in 1991. But the company cut that estimate to $900 million in April.

The company's capital budget this year is about $1.3 billion.

The reduction was necessary because the financial, real estate and travel markets are "uncertain," William J. Shaw, Marriott's chief financial officer, said in a statement.

Reflecting its diminished construction expectations, Marriott last week laid off 70 people in its architecture and construction division in the Washington area -- about 10 percent of that unit's staff.

Michael Mueller, a hotel-industry analyst with Montgomery Securities in San Francisco, said about the company's latest building plans, "I think it's a prudent move, given the current environment."

But he added, "This will make it difficult for Marriott to grow in the same way they have in the past. They have to look for alternatives" to building and selling hotels.

Also yesterday, Moody's Investors Service Inc., which rates the credit-worthiness of corporate and public debt issues, said it is reviewing about half of Marriott's long-term debt and is considering lowering its rating.

Late Monday, a rival service, Standard & Poor's Corp., made a similar announcement regarding some of Marriott's debts.

Moody's said its review was prompted by the "potential deterioration" of Marriott's profits and cash flow due to the soft travel market, which Moody's said could hamper Marriott's ability to cover its interest payments.

The other half of Marriott's borrowings, called commercial paper, is not included in either the Moody's or S&P reviews. Typically, a lower debt rating raises a company's cost of borrowing money.

Shaw said Marriott has had no trouble selling stakes in its hotels so far this year and did not expect problems in the balance of 1990. In fact, Chichester said a 1,500-room Marriott in San Francisco is expected to be sold within the next three months, reportedly for around $300 million. Moreover, Shaw said the company's cash flow remains strong, particularly because the company's contract-services businesses are unaffected by problems in the lodging industry. Marriott officials said they would discuss the company's operations with the ratings agencies.