By Rob Cameron
BBC News, Prague
An anti-corruption watchdog has asked Czech Prime Minister Stanislav Gross to explain how he paid for his luxury apartment in Prague five years ago.
Stanislav Gross said his uncle lent him the cash for the flat deposit
The Czech branch of Transparency International said Mr Gross' inability to explain the purchase was fuelling speculation over the money's origin.
It began when a newspaper claimed his earnings were too low to make a $50,000 (1.2m koruna) down-payment on the flat.
Mr Gross, who first said his uncle lent him the money, has threatened to sue.
The uncle initially confirmed Mr Gross' account, but under pressure changed his story, saying he himself had borrowed the money from relatives living abroad.
The daily paper Mlada Fronta Dnes said the uncle did not have enough money to lend the deposit on the flat, which has a terrace and a swimming pool.
The prime minister then accused the newspaper of underestimating his own earnings and threatened to sue.
Transparency International said Mr Gross' failure to give a satisfactory explanation was bringing the Czech government into disrepute.
Allegations of corruption are a perennial theme in the Czech Republic.
On Monday, a former finance minister lost his appeal in a high-profile corruption case and will spend the next five years in prison.