Top Financial Transaction Firms to Merge : Commerce: Companies, which authorize credit card payments and checks, agree to $6.6-billion deal.

Two companies whose businesses silently touch millions of consumers a day agreed Tuesday to a $6.6-billion merger.

Consumers will see no immediate impact from the merger of Atlanta-based First Financial Management Corp. and First Data Corp. of Hackensack, N.J., which authorize credit card payments and verify checks. But executives said the combined company would be in a better position to invest in new technology that could eventually speed up their clearance times.

The companies, each with annual revenue of about $2 billion, are the two largest in the industry, providing financial transaction services to banks, retailers, oil companies and corporations.

Both have been active acquirers of companies that handle electronic payments. In March, First Data acquired the industry’s No. 3, Melville, N.Y.-based Card Establishment Services Inc., and last year, First Financial bought Western Union’s money transfer business.

A number of new players, such as computer and telecommunications firms, have entered the field. By combining, First Data and First Financial will be better able to compete, executives said.

“We have the technology, we have the scale, and both companies have been innovative,” said Ric Duques, chairman and chief executive at First Data, who will continue as chairman of the new company.

The merger is subject to shareholder and regulatory approvals.

In the deal, shareholders of First Financial would receive about 1.59 shares of First Data stock for each of their First Financial shares. First Data’s shares fell 25 cents to $56.625 on the New York Stock Exchange on Tuesday. First Financial’s shares jumped $7.25 to $84 on the NYSE.