Downing Street did not give details, but Blair is expected to announce a date for his resignation after a decade as prime minister.
Blair's official spokesman said the prime minister would make a statement to Cabinet colleagues about his future as Labour Party leader. Blair is then expected to travel to his Sedgefield constituency in northern England to make a public announcement.
Speculation about Blair's resignation date has swirled for weeks, and intensified after he celebrated the landmark of 10 years in power on May 1.
In British parliamentary tradition, the leader of the largest party in Parliament serves as prime minister. Blair is expected to announce Thursday that he is stepping down as Labour leader, triggering a leadership contest that would see him replaced as prime minister within weeks.
Treasury chief Gordon Brown, Blair's longtime friend and rival, is the favorite to succeed him.