The EU Commission is stepping up its investigation of US chemical giants DuPont and Honeywell's business practices in developing automotive refrigerants.

Begun in 2011, the probe was ramped up a notch this week as the Commission sent the companies a formal statement of objections. It said the investigation panel's "preliminary view" was that the DuPont-Honeywell cooperative agreement begun in 2010 to produce the refrigerant HFO-1234yf may have limited availability and technical development of the product.

The partnership was established in reaction to the EU's adoption of new environmental standards for refrigerants in 2006, set down in the Mobile Air Conditioning (MAC) directive.

In filing its initial announcement of the antitrust probe, the Commission said it had received complaints from other companies that the two US players were preventing them from developing products with low global warming potential.

Both companies reacted to the Commission's announcements with statements of their own.

Thierry F.J. Vanlancker, president of DuPont Chemicals & Fluoroproducts, said his company will "vigorously defend" itself against the Commission's "allegations and preliminary conditions," arguing that it has "no basis in law or fact."

Without the cooperation, HFO-1234yf would not have been successfully developed and brought to market and there would today be no commercially available solution satisfying the MAC Directive's criteria, Vanlancker added.

In a separate statement, Honeywell said the Commission's objections were "baseless" and conflict with the EU's own laws encouraging collaboration on development.

EU rules permit companies to respond to the Commission's objections and request a hearing to present their arguments.