NEW YORK (Reuters) - The New York Times said on Sunday it made a mistake in charging a discounted rate for printing a political ad that attacked the top U.S. general in Iraq, angering Republicans and drawing charges the newspaper is politically biased.

Given the mistake, Moveon.org, the liberal anti-war group that purchased the ad on September 10 -- the day Gen. David Petraeus testified to Congress about the war in Iraq -- said on Sunday it was willing to pay the full price.

The ad, headlined "General Petraeus or General Betray Us," accused Petraeus of "cooking the books for the White House."

The New York Times' public editor Clark Hoyt wrote that in his opinion, not only did the advertiser get a discount it was not entitled to, but the ad violated The Times' own written standards.

"The ad appears to fly in the face of an internal advertising acceptability manual that says, 'We do not accept opinion advertisements that are attacks of a personal nature,"' he wrote, adding that the phrase "Betray Us" was "a particularly low blow when aimed at a soldier."

He quoted Times spokeswoman Catherine Mathis as saying the advertising representative who sold the ad failed to make it clear that for the standby rate of $65,000, The Times could not guarantee it would run it on the day of Petraeus' testimony.

That standby rate is offered to political and advocacy groups willing to be flexible about the day their ads run.

"We made a mistake," Mathis was quoted as saying.

Moveon.org said that it would wire the difference between the standby rate and the full rate of $142,083 to The Times.

"Now that the Times has revealed this mistake for the first time, and while we believe that the $142,083 figure is above the market rate paid by most organizations, out of an abundance of caution we have decided to pay that rate for this ad," it said in statement.