In a statement, Ascent said that its joint venture partners have been informed by the Slovenian Environment Agency that the permit is fully valid, and no further appeals against the permit are possible.
It added that the permit is crucially important. It allows the installation of a small processing facility, enabling gas to be sold into the national grid system.
The confirmation, therefore, marks an important step forward for the field development plan, it added.
"I am delighted that the IPPC Permit has been granted and we can continue to progress our plans to develop the Petišovci project to its significant potential," said Colin Hutchinson, Ascent chief executive.
Ascent shares rose by 15% to trade at 0.62p.