PRAGUE (Reuters) - Czech Prime Minister Bohuslav Sobotka will reshuffle his cabinet to boost his Social Democrat party's chances of winning national elections next year, he said on Friday, while stepping up a war of words with his main coalition partner.

The move, outlined in a letter sent to party members and seen by Reuters, comes four weeks after the center-left party lost regional and upper house elections to the ANO party.

"We need new blood. Therefore I have decided to change my team. On the level of the cabinet as well as party structures," Sobotka said.

The Social Democrats hold eight of 17 cabinet posts, while the ANO, a centrist movement led by Finance Minister Andrej Babis, has six. The coalition's third partner, the Christian Democrat party, has three.

Sobotka and Babis, a billionaire businessman and media owner, have operated an uneasy alliance since joining forces in January 2014, following national elections in October 2013.

In the letter, the prime minister sharply criticized the ANO and its leader, accusing them of waging a populist campaign against the Social Democrats.

"Our main competitor is not an equal political party but media-industrial conglomerate with huge amounts of money, loyal employees and indefinite media influence," Sobotka said.

While presiding over a growing economy and the lowest unemployment rate since 2008, the Social Democrats have been weakened by internal divisions, sparring with pro-Russian President Milos Zeman - a former Social Democrat chairman and prime minister - and being portrayed as part of a corrupt establishment by rivals including the ANO.

With parliamentary elections due in October, the ANO is leading the Social Democrats by up to 15 percentage points in opinion polls.

Babis, who owns the biggest private radio station and two national newspapers he bought before the last parliamentary election, has repeatedly denied taking advantage of his political position or media ownership to favor his business interests.

He is also the country's largest private employer with over 30,000 workers at his Agrofert group and other firms, spanning from farming to chemical production and healthcare.

Sobotka's reshuffle will only affect Social Democrat members of the cabinet. The party's ministerial portfolio includes industry, interior and foreign affairs.

(Reporting by Robert Muller and Jan Lopatka; editing by John Stonestreet)