* No details on where he is going, or why quitting country
* First rejected allegations, then apologised
* Future after penance to be decided with Vatican (Adds background, quotes, Mexican precedent)
By Philip Pullella
VATICAN CITY, May 15 Disgraced Cardinal Keith O'Brien, who resigned as head of the Roman Catholic Church in Scotland after admitting to sexual misconduct, will leave his country for months of "prayer and penance", the Vatican said on Wednesday.
A brief Vatican statement did not say where O'Brien, once Britain's most senior Catholic cleric, was going, or spell out why he was quitting Scotland.
But it will be hoping the announcement draws a line under an affair that has added to a sense of crisis in the Catholic Church as it continues to deal with separate scandals over sexual abuse of children by priests.
The cardinal resigned as archbishop of St. Andrews and Edinburgh on February 25 after three priests and one former priest in Scotland complained about incidents of sexual misconduct dating back to the 1980s.
O'Brien initially rejected the allegations published in a British newspaper and said he was seeking legal advice. But he later apologised for the misconduct.
The Vatican said on Wednesday his departure had been decided "in agreement with the Holy Father," stopping short of saying if Pope Francis had ordered O'Brien to go.
O'Brien would leave Scotland for "several months for the purpose of spiritual renewal, prayer and penance," the Vatican added.
He would be leaving for the same reasons that he decided not to participate in the conclave that elected Pope Francis on March 13, the statement said, without going into further detail.
At the time, O'Brien said he had ruled himself out of the conclave to avoid focusing media attention on himself.
Earlier this month Scottish media reported that Catholic leaders in Scotland had asked the Vatican to take action against O'Brien because his continued presence there would cause further scandal.
When he apologised for unspecified acts of sexual misconduct with adults, O'Brien said he had "fallen below the standards expected of me as a priest, archbishop and cardinal".
He also promised he would play no further part in the public life of the church in Scotland.
"Any decision regarding future arrangements for His Eminence shall be agreed with the Holy See," the Vatican added.
O'Brien's temporary exile, self-imposed or not, makes him the most prominent Churchman to withdraw for "penance" since 2006, when Pope Benedict ordered Father Marcial Maciel, founder of the Legionaries of Christ order of priests, to retire to a life of "prayer and penitence".
Maciel had for years contested accusations that he had abused seminarians and young men but Benedict forced him out when a Vatican investigation concluded the allegations were true.
Maciel died in 2008 and a year later the Legionaries were forced to admit that he had led a double life. Apart from having abused seminarians, Maciel secretly fathered children with at least two women, used drugs, and misused donations. (Reporting by Philip Pullella; Editing by Andrew Heavens)