President Lula da Silva pledged to rid Brazil of endemic corruption

Secretary General Silvio Pereira asked to be relieved of his duties while a parliamentary inquiry was carried out.

His decision comes a day before the party is due to hold an emergency meeting to discuss the growing crisis.

President Luiz Inacio Lula da Silva has been engulfed by the PT scandal, which has already claimed his chief of staff.

In his letter, Mr Pereira said he wanted to "devote himself fully" to his defence of the allegations, according to AFP news agency.

Tarnished image

The PT denies allegations that it offered bribes to secure political support in parliament.

But cabinet chief Jose Dirceu was forced to resign last month.

Brazilian weekly, Veja, published documents on Saturday allegedly showing contact between the PT and a financier accused of being at the heart of the scandal.

Analysts say Mr Pereira, as well as PT President Jose Genoino and treasurer Delubio Soares, may be forced to resign.

The allegations have tarnished the image of the PT, long viewed as a model of integrity.

President Lula da Silva won power in 2002, pledging to crack down on endemic corruption.