SOFIA (Reuters) - Socialist-backed candidate Rumen Radev, who has called for an end to European Union sanctions against Russia, has won the first round of Bulgaria's presidential election, the elections commission said on Tuesday, confirming preliminary results.

With 100 percent of polling stations counted, former air force commander Radev took 25.45 percent of the vote, followed by ruling center-right GERB party candidate Tsetska Tsacheva with 21.97 percent.

The two will now compete in a run-off on Nov. 13 with the 53-year-old Radev remaining favorite to win.

A Radev victory in the run-off could usher in months of political instability, including a possible snap parliamentary ballot, after Prime Minister Boiko Borisov signaled he may quit if his candidate Tsacheva loses.

(Reporting by Angel Krasimirov; Editing by Louise Ireland)