Outspoken American conservative Newt Gingrich has apologized for saying this week that some of the Sept. 11 hijackers entered the United States from Canada.

Gingrich, a former Republican speaker in the U.S. House of Representatives, retracted the comments on Wednesday after Canadian Ambassador Frank McKenna sent him a letter.

"Please accept my apology to the Canadian people for perpetuating the error; one I am sure that has been very painful to them," said Gingrich in a reply to McKenna.

Newt Gingrich, former Republican speaker in the U.S. House of Representatives. (AP file photo)

He told the ambassador he "deeply regrets" what has become a "widespread inaccuracy."

Speaking Tuesday on the Fox News political show Hannity & Colmes, Gingrich said: "Far more of the 9/11 terrorists came across from Canada than from Mexico."

None of the 19 hijackers entered the U.S. from either Canada or Mexico, information confirmed by former U.S. attorney general John Ashcroft and the 9/11 Commission investigating the attacks, wrote McKenna in his letter to Gingrich.

Frank McKenna (CP file photo)

The comments "perpetuate an urban legend that can take on a life of its own, especially when repeated by people whose opinions are deeply respected in the United States," he wrote.

"Canadians and Americans are great friends. And great friends can tell it like it is," he said. "In the interest of that friendship and to set the record straight, I ask that you retract your statement."