The leader of the 30-million-member National Association of Evangelicals in the US has resigned after being accused of paying for sex with a man.
Mr Haggard said he stepped aside "voluntarily"
The Reverend Ted Haggard said he would also temporarily step aside as head of his 14,000-strong New Life Church while his colleagues investigated the claims.
Mr Haggard, who is known as a vocal opponent of same-sex marriages, denies the accusations.
But a spokesman for his Colorado church said he had admitted "some guilt".
The row began after Mike Jones told a radio show in Denver, Colorado, that he had been paid to have sex with Mr Haggard nearly every month over the past three years.
In a statement to the New Life Church, Mr Haggard said he could "not continue to minister under the cloud created by the accusations".
"I am voluntarily stepping aside from leadership so that the overseer process can be allowed to proceed with integrity," he said.
In an interview with KUSA-TV, Mr Haggard said he "never had a gay relationship with anybody".
"I'm steady with my wife, I'm faithful to my wife," he said.
But the acting pastor of his church in Colorado, Ross Parsley, said Mr Haggard had made some admissions.
"There has been some admission of indiscretion," he said. "Not admission to all of the material that has been discussed, but there is an admission of some guilt."
The allegations come as voters in Colorado and several other US states are due to hold simultaneous votes on gay marriage laws.
Mike Jones said he had been paid to have sex
Mr Jones, 49, said Mr Haggard's public stance on gay marriage had prompted him to reveal the details of their relationship.
"It made me angry that here's someone preaching about gay marriage and going behind the scenes having gay sex," Mr Jones told the Associated Press news agency.
Mr Haggard, who has five children, became the president of the National Association of Evangelicals in 2003.
The preacher - who is also known as Pastor Ted - is one of America's most influential and politically well-connected religious leaders, says the BBC's Jane Little in Washington.
He has maintained close contacts with the White House and helped promote a conservative values platform.
His resignation comes as a blow for Republicans, who are hoping to energise a demoralised Christian base ahead of mid-term elections, our correspondent says.