Detroit Mayor Kwame Kilpatrick's chief of staff said Monday she is resigning amid allegations that she and the mayor lied under oath about an affair.

In a letter to Kilpatrick that was released by his office, Christine Beatty said she believes she can no longer effectively carry out her duties. Her resignation takes effect Feb. 8.

"I painfully regret the devastation that the recent reports have caused to the citizens of Detroit, to my co-workers, to the Mayor's family and to my family and friends," Beatty wrote in the letter.

Kilpatrick spokesman James Canning said the mayor's office had no comment.

There was no immediate response to messages seeking comment from Elliott Hall, a lawyer for Beatty.

A prosecutor launched an investigation last week into the allegations, which came to light when the Detroit Free Press reported details of steamy text messages between Beatty and Kilpatrick.

A conviction of lying under oath can bring up to 15 years' imprisonment.

Both Kilpatrick and Beatty testified in a trial last summer that they did not have a physical relationship in 2002 and 2003, when the messages were reportedly sent. The 14,000 messages examined by the newspaper reveal the two carried on a flirty, sometimes sexually explicit dialogue about where to meet and how to conceal their numerous trysts.

"I'm madly in love with you," Kilpatrick wrote on Oct. 3, 2002.

"I hope you feel that way for a long time," Beatty replied. "In case you haven't noticed, I am madly in love with you, too!"

On Oct. 16, 2002, Kilpatrick wrote Beatty: "I've been dreaming all day about having you all to myself for 3 days. Relaxing, laughing, talking, sleeping and making love."

Kilpatrick is married with three children. Beatty was married at the time of the messages and has two children. She has served as the Kilpatrick administration's chief of staff since the mayor took office in 2002.

The two, both 37, have been friends since they attended the same Detroit high school. Kilpatrick also appointed Beatty as his chief of staff when he became state House minority leader in 1999. She was his campaign manager during his campaigns for state House and the mayor's office.

Beatty wrote in her letter that her six years as chief of staff were the fulfillment of a childhood dream to serve her native Detroit.

"I've served the administration and Detroit citizens with diligence, strength and perseverance and I hope that my efforts will one day show through above all else," she wrote.

Beatty and Kilpatrick testified in a case involving a lawsuit filed by two police officers who alleged they were fired for investigating claims that the mayor used his security unit to cover up extramarital affairs.

The lawsuit ended with the jury awarding $6.5 million to the two officers. The payout eventually grew to more than $8.5 million.

The Free Press did not explain exactly how it obtained the messages. The newspaper said it cross-referenced the messages with the mayor's private calendar and credit card records to verify events in some of the notes.