A growing number of commercial mortgages and securities backed by pools of the loans are being sold on Wall Street and in Europe, a trend that could pour billions of dollars into the commercial real estate market within the next few years, bankers and brokers predicted this week.

About $10 billion to $15 billion worth of loans and commercial mortgage securities have been sold within the past 1 1/2 years, according to Philip Miller of the Mortgage Bankers Association of America (MBA).

Critics foresee obstacles to packaging the mortgages as well as risks to investors. One problem is that high vacancies in office buildings, running at a nationwide rate of about 15 percent, makes them poor investments in many cities. Although loans on hotels, shopping centers and other commercial properties also may be included in investment packages, office-building loans make up a large share of the pools in many cases.

Supporters of the infant secondary market for commercial loans, however, believe the obstacles can be cleared away and say some problems already have been solved. Robert Nau, managing director of real estate finance for Salomon Brothers, a major brokerage firm that has packaged several large groups of commercial loans, said "securitization" of commercial mortgages is "probably the biggest growth market on Wall Street."

Residential loans have been sold on the secondary market or packaged to back securities for about the last 15 years, with about $350 billion put into pools since 1970, according to the MBA. The secondary market has been a boon for the housing and lending industries because it generates money for new loans.

Growth of a similar market for commercial mortgages would benefit lenders, developers and institutional investors, such as pension funds and insurance companies, by generating as much as $500 billion for new loans, according to Ronald F. Poe, president of the MBA. "Without . . . a secondary market, lenders of commercial loans find they have to hold on to those loans until they're paid off," he said. The $500 billion represents the total of outstanding commercial debt. Poe is chief executive officer of Dorman & Wilson Inc. of White Plains, N.Y., which deals exclusively in commercial real estate lending.

The money from a thriving secondary market in commercial loans would probably lower interest rates by a half or full percentage point and provide enough capital for long-term financing for commercial borrowers, with terms of up to 20 or 30 years, Poe said. Most commercial mortgages now must be paid off in seven to 10 years.

But a flood of new capital for commercial development is just what that market does not need, according to Anthony Downs, a Brookings Institution economist. The commercial market is "overwhelmed with capital," and providing more money for more development would serve "no social need," he said.

An influx of money could fuel an increase in speculative building, which has been a big factor in producing the large number of "see-through," or nearly vacant, buildings in many American cities. Investments in overbuilt real estate markets also played a part in the widespread failures of savings and loan institutions in the last few years.

Poe said he believes the office vacancy rate will come down to a range of 5 percent to 6 percent from the present average of 15 percent across the country "if there is meaningful absorption over the next one to two years. This will probably happen if there is no overbuilding."

Downs, who said he does not believe creation of commercial mortgage-backed securities is a bad idea, just unneeded, said lack of standardization in commercial loans and absence of an encompassing rating system are obstacles that will have to be overcome before the secondary market can flourish.

Poe agreed that standardization and ratings are as yet unsolved problems. No standardized documents are used, and nearly every commercial mortgage has its own terms, prepayment provisions and other agreements.

More spadework has been done on ratings. Standard & Poor's, which rates stocks and bonds according to the degree of risk to the investor, established criteria for office buildings about a year ago, Poe said.

Standard & Poor's offers two types of office-building ratings, one based on the creditworthiness of the firm issuing the securities and another based chiefly on the projected cash flow from the properties in the pool, according to an MBA report. The first system is likely to get little use because "issuers who could qualify under this model already have the confidence of the investment community," the report said.

Many commercial mortgages now in existence cannot meet the standards for a rating based on the property, it said. To earn an A, considered a good classification, a property must be at least three years old and produce net revenue slightly higher than the annual principal and interest payments. A relatively small number of investments earn S&P's top ratings of triple-A and double-A.

Most of the loans sold or pooled so far have been "seasoned" loans at least three years old, which have a history of payment, Miller said. In many instances, the sales were "efforts by lenders to cash in on the loans in their portfolios."

New loans represent more risk to investors, and must be beefed up by "credit enhancement" devices, such as letters of credit or some kind of insurance, Miller said. The need to look to a third party for credit enhancement will limit the growth of a commercial secondary market, he added.

"A means of demonstrating to investors that the value is actually there" in the commercial loans is needed, according to Miller. "The rating system Standard & Poor is working on" will be the kind of system "investors are accustomed to.

"We also need a major secondary market maker," Miller said. Proposed legislation to establish a Federal Industrial Mortgage Association died in a congressional committee last year and has not been resurrected since.

The largest issuers of residential mortgage securities are the Federal National Mortgage Association (Fannie Mae) and the Federal Home Loan Mortgage Corp. (Freddie Mac), which buy loans from lenders and package them. Both are federally chartered institutions, with the backing of the U.S. government. But there is no Fannie Mae or Freddie Mac for commercial mortgages.

The creditworthiness and reputations of firms issuing securities have been the major factors reassuring investors in many past offerings. The value of the property and the credit of American Express were adequate collateral for securities based on new loans covering a new headquarters building for the company in New York City, Miller said.

Salomon Brothers' Nau said S&P does its analyses of office buildings on "worst case" bases, taking into account factors such as the possibility tenants may not renew leases or that owners may have to offer free rent, money for interior work and other concessions to get new tenants.

"We are concerned about the vacancy rate," and take only mortgages on "top tier" structures when pooling office-building mortgages, he said. Nau expects rating criteria to be developed for apartment buildings and shopping centers, actions that "will cause the market to expand."

In the meantime, successful offerings will have to be like Prudential Insurance Co.'s $1.3 billion bond issue last December, which had a triple-A rating because the bonds were guaranteed by Prudential, or based on existing mortgages that have a track record of paying off. Proceeds of the Prudential bond sale were used to purchase commercial mortgages in the company's portfolio, according to Nau.