Gazprom has sent Ukraine’s Naftogas a huge €5.2 billion bill under a ‘take-or-pay’ clause that Ukraine has to pay even if it hasn’t imported the gas. This was part of a dreadful 2009 gas deal negotiated by Yulia Tymoshenko during the last Russian - Ukrainian 'gas war'.
When Yanukovych was elected president, not only did he not annul this 2009 gas deal, he set it in concrete when, by signing the so-called inter-state Kharkiv accord, he unconstitutionally gave away the use of the Sevastopol naval base and its facilities to the Russian Black Sea fleet for decades, in exchange for a small discount on the price of gas imported by Ukraine from Russia.
As commentators have mentioned, many other European countries have recently successfully renegotiated gas deals with Russia at the Arbitration Court of the Chamber of Commerce, in Stockholm.
Ukraine would have a good case - they could probably prove the 2009 gas deal between Gazprom and Naftohaz Ukrainy was made under duress both from the Russian side and from European countries who were being starved of gas at that time; Ukraine is a major transit route for Russian gas bound for European customers. This would be sufficient for the court to call for the deal to be renegotiated.
The problem for Yanukovych, of course, is that if the Arbitration Court were to rule in Ukraine's favour it would prove that Yulia Tymoshenko was forced into the deal under unacceptable duress - she could not then be accused of exceeding her powers for which she was sentenced to seven years in prison. The case against her would have been shown to be purely politically motivated.
Keeping Tymoshenko in prison is more important to Yanukovych that paying over-the-odds prices for Russian gas.
p.s. Some say Gazprom are unlikely to go to the Arbitration Court for their money [because they would lose?] and will try other methods....
P.s. If anyone has any doubts about the sadistic treatment being systematically being meted out to Yulia Tymoshenko and Yuriy Lutsenko they should listen to Lutsenko's wife Iryna's passionate, moving address in the Verkhovna Rada today...