Ukrainian opposition leader, Yulia Tymoshenko, has been charged with misusing state funds while serving as prime minister.
Ms Tymoshenko is suspected of misspending money Ukraine received from the selling of its carbon emission rights under the Kyoto Protocol.
She has denied the accusations, saying she is being targeted for standing up to President Viktor Yanukovych.
Ms Tymoshenko lost the premiership in March following a no-confidence vote after she lost narrowly to Mr Yanukovych in a presidential election run-off.
The BBC reports Ms Tymoshenko was one of the leaders of the 2004 "Orange Revolution", which saw Mr Yanukovich stripped of victory in a presidential election.
On Wednesday, the prosecutor-general's office said an investigation had been opened into Ms Tymoshenko's second term in office.
After being questioned, Ms Tymoshenko said the probe centred around the alleged misspending of 320 million euros ($US425 million) which the government received in 2009 for the sale of carbon credits.
In October, an audit of her cabinet's affairs ordered by the new government - which is allied to Mr Yanukovych - accused her of using the money to cover pension arrears when finances were stretched by the global financial crisis.
Ms Tymoshenko denied the money had been spent on pensions and insisted it was still at the disposal of the environment ministry.
She said the investigation against her was a witch-hunt, adding: "The terror against the opposition continues."
The government has denied the allegations and said the probe is a legitimate attempt to uncover corruption by the previous administration.