KIEV, Aug. 29 - Ukraine’s largest opposition group Wednesday called for an investigation into the circumstances of the recent acquisition by Rinat Akhmetov, the wealthiest Ukrainian, of a 39.7% stake in the country’s biggest fossil burning power generator.

Yulia Tymoshenko, a former prime minister who may return to power after the Sept. 30 election, pledged to reverse the deal, according to which Akhmetov had acquired the stake in DniproEnergo.