The ongoing battle for ownership of Dollar Thrifty has been stepped up once again.

The Avis Budget group has announced that it has raised the cash portion of its bid for the Tulsa-based Dollar Thrifty, intensifying its hostile bidding war with rival Hertz Global Holdings.

Yesterday, Avis declared that it would now be offering $40.75 per share in cash, an increase from its previous bid of $39.25. It has also announced the offering of a revised share component, which will see 0.6543 Avis shares offered for each Dollar Thrifty share, taking the total value of the bid to $1.35 billion. This represents a significant increase on the initial offer which was made by Hertz way back in April, valued at $1.1 billion.

The deal, which has dominated the car hire industry since the bidding war began, is still under regulatory investigation, with US antitrust officials still deliberating on whether a successful takeover of Dollar Thrifty by either of its larger rivals would create an unfair weighting of the market. Dollar Thrifty is highly valued by its suitors for its presence in the budget or value market, which would compliment both the existing portfolios of Hertz and Avis.

Dollar Thrifty, has so far resisted the advances of Avis, with some analysts suggesting that the Hertz contract, signed in principle when first tabled, contained numerous, costly clauses that prevented Dollar Thrifty from considering other offers. The smaller Oklahoma organization only last month that a higher offer, which was anticipated from Avis, would have no bearing on the ultimate decision, although shareholders in the group have voiced concerns otherwise.