By Philip Pullella

VATICAN CITY (Reuters) - The scandal-plagued Legionaries of Christ Roman Catholic religious order suffered another major blow to its image on Tuesday when one of its most prominent members admitted to having fathered a child.

Father Thomas Williams said in a statement he was "truly sorry to everyone who is hurt by this revelation".

Williams, the American author of more than a dozen books on spirituality and dean of moral theology at a papal university in Rome, often appeared on American television to explain Vatican affairs and Catholic teachings.

The Legionaries have been at the centre of controversy since 2009 when they admitted their charismatic Mexican founder, Father Marcial Maciel, had led a double life, secretly fathering children, using drugs and misusing donations.

Maciel was also accused of sexually abusing seminarians.

He had enjoyed the support of the late Pope John Paul and was spared official censure for years. Pope Benedict ordered him to retire in 2006 to a life of "prayer and penitence."

"A number of years ago I had a relationship with a woman and fathered her child. I am deeply sorry for this grave transgression and have tried to made amends," Williams said in his statement.

Father Luis Garza, the Legionaries' head for North America said in a letter to members that Williams would undergo a period of "reflection, prayer and atonement without public ministry".

The Legionaries of Christ runs private Catholic schools and charitable organizations in 22 countries via its network of 800 priests and 2,600 seminarians. The order's lay movement, known as Regnum Christi, has around 75,000 members.

Last week the Vatican said its doctrinal department was investigating seven Legionaries for suspicion of having sexually abused children.

(Editing by Andrew Roche)