LONDON—Tony Blair will undergo a public grilling this week over the U.K.'s role in the Iraq war, raising questions about the former prime minister's legacy and his value to the Labour Party he once led as it prepares for a tough election battle.

In a much-anticipated appearance on Jan. 29 before a five-member panel investigating the war, Mr. Blair is expected to face questions about the legitimacy—and even legality—of the U.K.'s involvement in Iraq. He will be confronted about whether he committed to overthrow Saddam Hussein long before the immediate run-up to the war, and quizzed about criticisms of the U.K's preparedness for the invasion and its minimal influence over U.S. allies.

The spotlight on Iraq is the latest in a series of dents Mr. Blair's reputation has sustained since leaving office in 2007 after a 10-year run. One of modern Britain's most influential and popular politicians, he led the seemingly unelectable Labour Party to power in 1997.

Now, however, the war and Britain's deep recession have soured voters on the Labour regime. The former prime minister has taken a series of hits: the rejection by European governments of Britain's attempts to install Mr. Blair as the European Union's first president, questions about his effectiveness as a U.N. special envoy to the Middle East, and his acceptance of lucrative consulting work from companies such as the U.S. bankJ.P. Morgan Chase & Co., leading to accusations—including from within his party—that he has cashed in on his status. He also makes paid speeches, including a series of talks he is due to give to London hedge fund Lansdowne Partners Ltd.

Mr. Blair has long said it was right to remove Iraqi leader Saddam Hussein because of the threat he posed to the region. A spokesman for Mr. Blair declined to comment on the coming hearing.