Russian prosecutors have opened a criminal inquiry into allegations of fraud involving the country's former prime minister, officials have said.
Mr Kasyanov was highly critical of the Yukos investigation
Mikhail Kasyanov has denied any wrongdoing, saying he always obeyed the law while in office.
Analysts say the case may be linked to Mr Kasyanov's political aims, as he has hinted he may run for president.
Mr Kasyanov has been highly critical of Vladimir Putin since the president dismissed him in 2004.
Mr Kasyanov is under investigation for acquiring land fraudulently.
He has strongly denied being involved in any improper deals.
"Through all my years in government service, I never founded any commercial organisations and never held any stocks, bonds, interest or equity for any kind of firms," he said in a statement issued by his spokeswoman.
Mr Kasyanov, 47, was sacked without explanation by President Putin a year ago.
As prime minister he was one of the last survivors of the Boris Yeltsin-era camp at the Kremlin.
Analysts said his dismissal last year was linked to a disagreement over the clampdown on Russia's largest oil company, Yukos, and the imprisonment of its main shareholder, Mikhail Khodorkovsky.
Since then Mr Kasyanov has criticised the course of the Russian government. He is now seen as the future leader of the country's liberal opposition.
He has said the Yukos investigation has damaged the country's economic development.
He has also strongly hinted that he is considering standing for the presidential elections in 2008, when Mr Putin's term ends.
The BBC's Steven Rosenberg, in Moscow, says that Mr Kasyanov's supporters believe the reports of a criminal investigation may be designed as a warning to encourage him to stay out of politics.
The probe began after a request from an investigative journalist who is a member of the anti-corruption committee of the lower house.
Russia's courts are widely regarded as being under Kremlin control, although the Kremlin says the courts are independent of the executive.