FILE PHOTO: U.S. Senator Al Franken walks out to speak to the media outside his office on Capitol Hill in Washington, U.S. on November 27, 2017. REUTERS/Yuri Gripas/File Photo

WASHINGTON (Reuters) - U.S. Democratic Senator Al Franken will announce his resignation on Thursday, a day after a majority of his Democratic Senate colleagues called for him to step down following a string of sexual misconduct allegations against him, CNN reported on Thursday, citing unnamed sources.