Former Ukrainian prime minister Yulia Tymoshenko has been jailed for seven years after a trial condemned by the EU and US as politically motivated.
Tymoshenko, the country's main opposition leader, was convicted of abuse of office by a court in the capital Kiev.
The European Commission said it was deeply disappointed by the sentencing and claimed it could have profound implications for ties with the EU.
EU diplomats warned Ukraine that planned bilateral agreements on political association and free trade may never be ratified if Tymoshenko ends up in jail.
Or if she is barred from running for office before parliamentary elections next year, they added.
The 50-year-old was found guilty of violating legal procedures during the signing of a contract to import natural gas from Russia in 2009.
Tymoshenko has dismissed the trial as an attempt by her arch-foe President Viltor Yanukovych to bar her from upcoming elections.
Judge Rodion Kireyev said she caused damages of 1.5bn hryvna (£121m) to the national gas company by signing the contract with Moscow two years ago.
Tymoshenko, wearing her trademark blond braid wrapped around her head, looked composed in the courtroom, occasionally chatting with her daughter Eugenia as Mr Kireyev spoke.
Occasionally, the defendant even talked to reporters while Mr Kireyev read out the lengthy ruling, causing him to become visibly irritated.
During a break before she was found guilty, she said: "Whatever the verdict will be ... I will continue my fight for Ukraine, for its European future.
"Nobody, not Yanukovych , not Kireyev, can humiliate my honest name. I have worked and will continue to work for Ukraine's sake."
Prosecutors said she was not authorised to order the signing of the natural gas contract.
And they claimed the price she agreed to was too high, causing losses to the state budget.
Tymoshenko said as a prime minister she did not need any special permission to order the signing of the deal.
She insisted her actions helped end a bitter pricing dispute between Moscow and Kiev, which had lcaused energy supply shortages across Europe.
The defendant had already been jailed for more than two months during the trial for contempt of court.
Tymoshenko helped to lead the 2004 Orange Revolution, which overturned Mr Yanukovych's fraud-tainted election victory.
But he staged a comeback, narrowly defeating Tymoshenko in a 2010 presidential vote amid public anger over economic problems.