WASHINGTON - A bipartisan group of lawmakers has asked U.S. antitrust regulators to reopen their investigation into CVS's 2007 merger with pharmacy benefit giant Caremark.

Eight congressmen in a letter to the agency that approved the merger, the Federal Trade Commission, accused the drugstore chain CVS of arranging to have consumers' prescriptions filled only at CVS without the consumers' permission and using Caremark data on consumers to steer them to CVS pharmacies.

They also accused CVS of designing its prescription drug card to mislead consumers into believing it could only be used at CVS.

We are troubled that these are unfair and deceptive business practices. We strongly encourage the FTC to reopen the CVS Caremark merger investigation and determine if the acquisition poses a threat of reducing competition or whether CVS is engaging in any unfair or deceptive practices, said the letter dated Monday.

Four Democratic congressmen -- Anthony Weiner, Marion Berry, Michael Arcuri and Lloyd Doggett -- and four Republicans -- John Boozman, Mike Rogers, Walter Jones and Robert Aderholt, signed the letter.

The labor group Change to Win, made up of the Service Employees International Union and six other unions, has also asked for the merger to be revisited on the grounds that the deal has led to higher drug prices.

Five U.S. senators wrote to the FTC in July to ask the agency to look into allegations that CVS inappropriately used its pharmacy benefits business to win clients and squeeze smaller competitors. They stopped short of asking for a reassessment of the merger.

Previously, independent pharmacists asked for a firewall to be erected between CVS's retail pharmacy business and its pharmacy benefits business. CVS has more than 6,900 stores.

The FTC declined to comment. CVS did not return a call seeking comment. (Reporting by Diane Bartz, editing by Leslie Gevirtz)