Dollar Thrifty has sent a letter to both Hertz Global Holdings and Avis Budget Group seeking “best and final” merger proposals.

Dollar Thrifty has stipulated a deadline of October for submissions.

Scott Thompson, Dollar Thrifty president, said: “Hertz and Avis have made substantial progress with respect to regulatory reviews and our board has concluded that the time has come to determine if, and on what terms, a transaction can be achieved.

“Continuing uncertainty is in no one’s best interest and both Hertz and Avis have the information they need to put forward their best and final offers.”

Earlier this month, the Federal Trade Commission asked Hertz for additional information regarding its proposed takeover of Dollar Thrifty.

Avis is still seeking antitrust approval for its offer.

With just four major competitors in the American car rental market - Hertz, Avis, Dollar Thrifty and industry leader, privately held Enterprise – antitrust concerns have become a dominant factor in any merger.

Hertz is presently in pole position to secure the deal, having offered close to $2.1 billion for Dollar Thrifty in May, taking advantage of Avis’ problems getting regulatory clearance for a rival bid.

Thompson added: “We are optimistic about our future but it’s clear that bringing closure one way or another to the process with Hertz and Avis is in the best interests of our shareholders, the company and our employees.”