LONDON — By tradition, former British prime ministers are honored by Queen Elizabeth some years after leaving 10 Downing Street, so the elevation of Tony Blair to a knighthood on New Year’s Day could have been a routine event.
Instead, more than 600,000 people have signed an online petition asking that the honor be rescinded, illustrating how one of Britain’s most successful politicians remains a divisive figure, never forgiven by his critics for taking the country to war in Iraq.
The petition, which has no legal force, says Mr. Blair, 68, was “personally responsible” for causing the deaths of countless civilians and service members in “various conflicts,” adding that “he should be held accountable for war crimes.”
That sentiment reflects the extent to which the legacy of the opposition Labour Party’s most electorally successful recent leader has been defined by his staunch support for the United States and President George W. Bush in a war in Iraq that became steadily more unpopular in Britain.
The petition comes after Mr. Blair became a member of the Order of the Garter, following an appointment made on New Year’s Eve by the queen. Dating back almost 700 years, the order is the oldest and most senior Order of Chivalry in Britain. The current prime minister, Boris Johnson, was not involved in the decision, his office said.
Mr. Blair stepped aside as prime minister in 2007, became a Middle East peace envoy, set up a foundation and has recently made a number of well-received suggestions about how to handle the pandemic.
But the speed with which the petition went viral illustrates that his efforts at political rehabilitation have had only limited success.
“It’s a fool’s errand,” said Steven Fielding, professor of political history at Nottingham University. “His reputation for the moment is locked into the Iraq war. He spent the last year of his premiership trying to establish his legacy, and it was all pointless because his legacy, like it or not, was Iraq, and is Iraq, and will remain Iraq.”
Mr. Blair led his Labour Party to a landslide election victory in 1997, was prime minister for a decade and won two more general elections along the way. He pulled Labour toward the political center, helped negotiate a peace agreement in Northern Ireland and presided over a generally healthy economy spending buoyant tax revenues on health and education.
But after leaving Downing Street, Mr. Blair became a wealthy man and advised some foreign governments and multinational companies, reinforcing his reputation as someone who liked to cultivate the rich and powerful.
He is disliked by those on both wings of the political spectrum: Those on the left accused him of betraying his party’s values by shifting Labour to the center, while those on the right resented his formidable electoral appeal.
“In a way the hate is in fact a tribute to Tony Blair,” Professor Fielding said. “If he could see it like that he would almost enjoy it, but clearly it’s painful to him.”
Of the other four living former prime ministers, only John Major has received a knighthood-level honor, though Gordon Brown, David Cameron and Theresa May served in Downing Street after Mr. Blair.
Despite its support, the online petition is unlikely to succeed in its aim of rescinding Mr. Blair’s knighthood, and the current leader of the Labour Party, Keir Starmer — who has also been trying to shift the party to the political center — defended the decision to award it.
“I don’t think it’s a thorny issue at all for me, I think Tony Blair deserves the honor,” Mr. Starmer told ITV, citing achievements including Mr. Blair’s work on the Northern Ireland peace process. “He won three elections, he was a very successful prime minister.”
In a speech on Tuesday, Mr. Starmer also praised Mr. Blair’s government, saying that it had “introduced a national minimum wage and repaired the public services that had been neglected under the Tories.”
And there was also support from a government minister, Maggie Throup, who told LBC Radio that Mr. Blair “did lots of good things,” adding: “I think it’s only right that we do honor our previous prime ministers.”
Professor Fielding said that, historically, it is not uncommon for successful prime ministers to suffer big drops in popularity after relinquishing power, and it has often been decades before their records are reassessed more evenhandedly.
That time has does not yet seem to have arrived for Mr. Blair, and for some of his critics, it probably never will. Among the signatories of the petition is Twiggy Garcia, who, while working part-time in a restaurant in 2014 where Mr. Blair was dining, attempted a citizens’ arrest on the former prime minister, alleging war crimes.
“I think it is a bit of a disgrace that the queen is looking to knight him,” said Mr. Garcia, a D.J., who is unimpressed by arguments that Iraq should not detract from Mr. Blair’s other achievements.
“I think the good things Tony Blair did for this country shouldn’t detract from the crimes that he committed and how we view them,” added Mr. Garcia, who, when asked if he would still attempt an arrest on the former prime minister, replied: “One-hundred, percent I would do it again.”