* Legionaries head asks forgiveness for mishandling case

* Scandal concerns popular American priest who fathered a child

* Fr. Thomas Williams was allowed to continue teaching, appearing on TV

By Philip Pullella

ROME, May 22 (Reuters) - The leader of the Legionaries of Christ admitted on Tuesday he knew for years the scandal-plagued religious order's most famous priest had fathered a child but still allowed the popular cleric to preach about morality.

The religious order, still reeling from revelations that its founder was a sexual abuser and drug addict with two secret families, suffered another major blow last week when it admitted that Father Thomas Williams, an American based in Rome, had also led a double life.

But the question that had been left hanging after the first admission was how long Williams' superiors knew.

In a letter to members published on the order's website, Father Alvaro Corcuera said he found out about Williams's child "early in my new assignment" as the order's director-general, which began in 2005.

In an interview with Reuters last week, Cardinal Velasio De Paolis, the man Pope Benedict appointed in 2010 as his personal delegate to try to reform the order, said he did not find out about Williams' affair until this year.

Williams was the public face of the order, appearing often on American television networks to explain Church teachings. He was the author of more than a dozen books, including one called "Knowing Right From Wrong: A Christian Guide to Conscience".

He was a big draw on the lecture circuit at Catholic institutions and had two websites, both of which were shut down last week after the order issued a statement about him.

In his letter to members, Corcuera said that after he first found out that Williams had a child he asked him to "start withdrawing from public ministry" but admitted that the restrictions "were not firm enough" and Williams was allowed to continue teaching.

In fact, Williams continued to appear in public and teach at Rome's Pontifical Regina Apostolorum University.

He appeared as a consultant to a U.S. television network until 2010 and had a contract with the network until May 2011, when it was not renewed, according to a source with knowledge of the matter.

Corcuera said in his letter that he did not give Williams an "explicit indication to full withdraw from all public ministry" until March 2012. By that time it was an open secret to a number of television journalists.

In the letter, Corcuera "begs" forgiveness from members of the order for being an ineffective leader and for not being "diligent in setting proper restrictions and enforcing them" in the Williams case.

FOUNDER LED DOUBLE LIFE

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

The order and its leaders have been at the centre of controversy since 2009 when they were forced to admit that their charismatic Mexican founder, Father Marcial Maciel, had led a double life for decades.

Maciel, who made huge financial contributions to the Vatican, secretly fathered children with at least two women, used drugs, misused donations and sexually abused seminarians.

He had enjoyed the support of the late Pope John Paul and was spared official censure for years despite what critics say was overwhelming proof of his crimes.

Some Churchmen say the scandal may delay John Paul's road to sainthood. Last year he was beatified, the last step before being made a saint.

Pope Benedict ordered Maciel to retire in 2006 to a life of "prayer and penitence" when the evidence could no longer be disputed, and he died in disgrace in 2008.

Some critics of the order say that since the Legionaries have such a history of scandal and cover-up that they are beyond repair and should be shut down, with good priests joining other religious orders or be put under control of bishops.

Corcuera said the Legionaries now had put in place new procedures to act quickly "when a serious charge is brought against any Legionary" including possible immediate removal from public ministry.

Seven members of the order are being investigated by the Vatican's doctrinal department for sexual abuse of minors. (Reporting By Philip Pullella; Editing by Jon Hemming)