Noppadol Pattama said Thaksin, who was ousted in a bloodless military coup last September, would not return to answer charges of corruption and abuse of power because he fears for his safety.
Thai authorities gave Thaksin until the weekend to return to the country to face charges of illegal ownership of shares from the Thai stock exchange worth millions of dollars.
Prosecutors said that if he fails to meet the deadline, they would issue an international arrest warrant for him and seek his extradition from the U.K., where he currently lives.
He has been charged with improper disclosure of corporate information by failing to report his ownership and sale of shares on the Thai stock exchange.
Thaksin became a billionaire in telecoms before going into politics in 2001-2006.