Pope Benedict XVI has apologised to victims of child sex abuse by Irish clergy, and announced a formal probe at religious institutions in Ireland.

In an unprecedented letter addressing the abuse of minors in Ireland, the pontiff says: "You have suffered grievously and I am truly sorry". He adds he feels "shame and remorse".

In the letter, the Pope also announced a formal Vatican investigation of Irish dioceses, seminaries and religious orders affected by the scandal. He said he was willing to meet victims to acknowledge their suffering personally.

Pope Benedict accused Irish bishops of making "serious mistakes" in responding to allegations. He warned priests and religious workers guilty of child abuse they "must answer" for their crimes "before properly constituted tribunals."

Over the past five years, predominantly Catholic Ireland has been shocked by three judicial reports revealing ill-treatment, abuse and cruelty by clerics  and a cover-up of their activities by church authorities. The latest revelations, compounded by evidence of a cover-up by the church hierarchy, shook Ireland late last year.

Since then, new abuse scandals have come to light in the Pope's native Germany, Austria, the Netherlands and Switzerland. The Pope says he hopes his letter will "help repentance, healing and renewal".

Irish victims of clerical child sex abuse say they are deeply disappointed by the Pope's letter of apology. A group representing victims says the letter focuses too narrowly on lower-rank Irish priests without recognising the responsibility of the Vatican and senior Irish clerics for protecting offenders.

Read more: Pope shocked by sexual abuse