James Marson in Kyiv -

From a hot, packed Kyiv courtroom on Friday, June 24, Ukrainian opposition leader Yulia Tymoshenko fired a warning shot at her long-time rival, President Viktor Yanukovych. "My voice will be even louder from prison because the whole world will hear me," she said during a day of verbal sparring with the judge at her pre-trial hearing.

The trial, where the former prime minister and Orange Revolution leader will stand charged of abuse of power over a natural gas deal with Russia, starts on June 29, and will be the first of three cases against Tymoshenko to reach court. Prosecutors allege that she abused her power in concluding a gas deal with Russia in January 2009 that locked Kyiv into a gas price formula linked to the price of oil, which has sent the country's gas payments spiralling in the past few months.

Tymoshenko's theatrical performance at the hearing - the court was "a farce," the judge "a puppet" - highlighted the dilemma that Yanukovych, who denies any political motivation to the charges, is now facing.

The temptation to lock up his great rival and throw away the key must be great. Despite losing the presidential election to him in early 2010, Tymoshenko remains the most powerful opposition figure and her party is the greatest threat to Yanukovych's Party of Regions in next year's parliamentary ballot. But there are two major reasons that could hold the authorities back from sending Tymoshenko down.

Proceed with caution

First, it could turn her into a martyr and galvanize her support, as when she was jailed briefly under former president Leonid Kuchma in 2001. Tymoshenko is at her best in a scrap. Second, the West has given Yanukovych plenty of warnings to proceed with caution in the trial of Tymoshenko and around a dozen other former members of her government currently under investigation. With the EU and Ukraine close to agreeing on an association agreement, including a deep free-trade agreement, any decision to jail Tymoshenko could throw a spanner in the works. The Americans have also voiced concerns, including a telephone call from Vice President Joe Biden in March.

Yanukovych needs the West as a counterweight to Russian influence in Ukraine. Following the deal last year to extend the stay of Russia's Black Sea Fleet in a Ukrainian port until 2042 in exchange for a cut in the gas price, the Kremlin has pushed hard for further concessions - for instance, a merger of the countries' state gas companies, or a stake in Ukraine's gas transport system.

From the very start, the authorities have done their best to convince the West that the probes into Tymoshenko and her former colleagues have nothing to do with politics and are part of a wide-ranging crackdown on corruption. Few people in Kyiv take that seriously.

The public relations battle was kicked off by the authorities last October when a $2m report into alleged corruption in Tymoshenko's government was published by three top US legal and investigative firms. Tymoshenko hit back in June with a report by Covington & Burling, a US law firm, and BDO, an accounting firm, saying they had found no evidence of wrongdoing in the other two cases, concerning the purchase of medical vehicles and the use of carbon credits. She has also complained to the European Court of Human Rights about her treatment.

The current government doesn't miss an opportunity to use the case for political PR as well, blaming the country's continued economic woes on this gas deal. But this has failed to shift the blame entirely: Yanukovych's approval ratings are drifting towards single figures as inflation continues to bite. This must make the temptation to take Tymoshenko out of the picture all the more tempting.

Some analysts suggest that handing her a suspended sentence is the most likely outcome, which would see her walk free, but barred from the parliamentary elections next year. The next few weeks are sure to be a hot and uncomfortable time, not only for Tymoshenko in the courtroom, but also for those who are deciding her fate.