PRAGUE (REUTERS) - Former Czech prime minister Stanislav Gross, who became the country's youngest ever premier more than a decade ago, has died at the age of 45 after a long illness, a Social Democrat party spokeswoman said on Thursday.

Gross served as interior minister in two governments, including under Milos Zeman, who is now president, and then became prime minister in July 2004 at the age of 34, leading a Social Democrat government.

Less than a year later, he left office amid a scandal over the source of funds he used to buy a Prague apartment.

Gross later returned to law and also invested in a Czech electricity company, raising more questions from the media over his finances although an anti-corruption probe found no wrongdoing.

Czech media have reported in recent months that Gross was battling amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig's disease.