The Vatican has made public its displeasure at the decision of a retired bishop to run in Paraguay's 2008 presidential elections.
Fernando Lugo is known for his work among the poor
Papal officials released the text of a letter handed last week to Fernando Lugo warning him of possible sanctions.
Mr Lugo said on Monday he hoped to lead an alliance against President Nicanor Duarte and had left the priesthood.
The Pope "can either accept my decision or punish me. But I am in politics already," the 55-year-old said.
'Figure of unity'
The Vatican letter, signed by Cardinal Giovanni Battista Re and made public on Tuesday, is dated 20 December, one day before Mr Lugo presented his resignation to the papal authorities.
"In the name of Jesus Christ, I ask him to seriously reflect about his behaviour," the letter reads.
"The Holy See has learned with surprise that some political parties have the intention of presenting him as candidate in the coming presidential election in Paraguay.
"The acceptance of that offer would be clearly against the serious responsibility of a bishop... Canonic Law prohibits priests from participating in political parties or labour unions."
Known for his work among the poor, Mr Lugo is leading in the opinion polls.
Many believe he is the only figure who can unite the opposition to defeat the conservative Colorado Party, which has been in power for about 60 years.
Mr Lugo was appointed bishop of San Pedro by Pope John Paul II in 1994. He retired as bishop 10 years later.