Dollar Thrifty Automotive Group has announced an end to its quest for a corporate merger, saying that none of the proposals received so far would eliminate the antitrust regulatory risk, and would likely not be acceptable to its shareholders. “It is time for us to move forward on a standalone basis,” said Dollar Thrifty CEO Scott L. Thompson.

Dollar Thrifty’s announcement ends a year-long battle between Hertz and Avis Budget to acquire Dollar Thrifty. In the end, Avis hit a regulatory roadblock, leaving the way clear for Hertz with its offer of $1.91 billion, higher than the $1.55 billion bid by Avis.

“While Hertz’s May 2011 exchange offer remains outstanding, and on Oct. 7, 2011, Hertz’s CEO called me personally to reaffirm their commitment to pursuing the acquisition of Dollar Thrifty, the fact remains that they have not made a proposal that addresses our board’s requirements,” Thompson said. “Having received no acceptable offer after conducting an unprecedentedly open process with clearly articulated requirements, it is time for us to move forward on a standalone basis.”

For its part, Hertz said that it is still interested in acquiring Dollar Thrifty and is trying to get a consent decree from the Federal Trade Commission. Dollar Thrifty’s standalone plan includes a buyback of as much as $400 million of its stock.