A new row erupted on Thursday in the murky east European gas trade when Dmitry Firtash, the Ukrainian gas billionaire, claimed he had lost control of a key subsidiary through the fraudulent actions of a renegade employee.

The dispute over the ownership of Emfesz, a big Budapest-based gas and energy company, comes weeks after Mr Firtash was squeezed out of the Ukrainian gas trade under the terms of a new Russian-Ukrainian contract signed after the January gas supply crisis.