A long-simmering feud between a global real estate services giant CBRE Group and District-based CoStar is beginning to boil over.

A CBRE executive last month sent out a memo barring brokers from sharing individual transaction information with CoStar, depriving the data firm of a sizable amount of information about commercial real estate transactions.

CoStar founder and chief executive Andrew C. Florance returned fire, saying in an interview that the federal government should examine CBRE for anti-competitive business practices.

At issue is the lifeblood of CoStar’s business, and an increasingly valuable commodity in an industry dominated by fewer and fewer big players — the often-private details about how much people are paying to lease office buildings.

On Dec. 9, Christopher R. Ludeman, CBRE president of capital markets, e-mailed a directive with the subject “CoStar” to company brokers reminding them not to share transaction-specific details with CoStar. The memo was not meant to be shared outside the company, but was obtained by Capital Business. In it, Ludeman said CoStar “has becoming increasingly creative in attempting to induce us to release transaction details to them” via ranking categories such as “Power Brokers.” Ludeman calls such rankings “illegitimate.”

“Your clients, the media and the local business community at large should be advised that the CoStar rankings are inherently flawed, and not at all reflective of market position or individual professional achievement,” Ludeman wrote.

In an interview, Florance called Ludeman’s order another step toward a hoarding of data that he said was damaging to the real estate industry and ought to be examined by the federal government for anti-competitive practices.

The commercial real estate brokerage business, he said, “is the most concentrated industry that exists.”

“They’re trying to hide more and more pricing data, because there is a smaller and smaller number of [brokerage] companies trying to corner all the data,” Florance said of CBRE.

When CBRE conceals market data, Florance argued, it forces landlords and tenants who might otherwise use alternative brokers or conduct their own deals to consider CBRE. “The net effect,” he said, “is size begets size.”

“One thing that could happen is that owners lose the ability to represent themselves because they lack the necessary information,” he said.

CoStar has its own issues with industry consolidation. Its $860 million acquisition of the real estate listing service LoopNet is under antitrust review. On Jan. 4 the two firms agreed to extend a waiting period to consider what if any concessions they might have to make to get the green light from regulators, according to a filing with the Securities and Exchange Commission. Florance declined to comment on the progress of the negotiations.

Robert McGrath, CBRE senior director of corporate communications, said in an e-mail that CBRE has for years had a policy of not sharing client leasing information “with any third-party database provider, including CoStar.”

“Much of our client information is confidential, and we are obligated to closely guard this information,” McGrath wrote.

CBRE, publicly traded and based in Los Angeles, is one of the largest brokerage firms in the world, but research differs as to the extent of its dominance. According to data on more than 3,000 brokered U.S. sales through the first three quarters of 2011 from Real Capital Analytics, CBRE had a 14.3 percent market share. Florance said he believes CBRE’s share of the total market is much larger; CoStar is compiling its own data on around 50,000 commercial sales from 2011.

But McGrath said the industry was anything but overly consolidated. “Our clients expect us to have the best possible market intelligence. As such, our proprietary database of deal information is a distinct competitive advantage in a highly fragmented industry,” he said.