By Nate Raymond and Braden Reddall

NEW YORK/SAN FRANCISCO (Reuters) - A former Ecuadorean judge has claimed that after stepping down from the bench, he illegally ghostwrote a judgment in which Chevron was ordered to pay $18.2 billion (11.5 billion pounds) for polluting the rain forest, and that the plaintiffs paid a $500,000 bribe to the judge who issued the ruling.

Alberto Guerra, who presided over the case from 2003 to 2004, made the allegations in a sworn statement filed by Chevron on Monday in support of a lawsuit in Manhattan federal district court accusing the Ecuadorean plaintiffs and their lawyers of fraud.

"Another participant in the fraud has now come forward rather than wait to be exposed by others," Hewitt Pate, Chevron vice president and general counsel, said in a statement.

Karen Hinton, a spokeswoman for the plaintiffs, in a statement called Guerra a "disgraced former Ecuadorean judge who is being paid hundreds of thousands of dollars by Chevron to make false allegations about the Ecuador trial court judgment."

Guerra was not available to comment.

The filing is the latest in an ever-escalating two-decade battle between Chevron and residents of Ecuador's Lago Agrio over oil extraction in the region.

Residents claimed that Texaco, which Chevron acquired in 2001, polluted the rainforest and water supplies with hundreds of unlined waste pits from 1964 to 1992, damaging crops and public health including deaths from cancer.

Chevron claimed that its share of the waste pits had been cleaned up and that its activities were not responsible for environmental and public health damage.

An Ecuadorean court entered the $18.2 billion judgment in 2011. The award was upped to $19 billion in July.

The National Court of Justice, Ecuador's highest court, last month appointed three judges to hear Chevron's final appeal in that country of the court decision.

One of them, Oscar Bermudez, told Reuters that they are not allowed to consider new evidence.

"Our job is to make sure the sentence issued by the previous court complies with legal and constitutional precepts ... they can provide new evidence but the court cannot take it into account," he said, adding that Chevron may consider presenting Guerra's statement to Ecuador's attorney general for his consideration.

Chevron, with no assets in Ecuador now, has fought a global campaign against the enforcement of the $19 billion award. In February 2011, it filed the New York federal lawsuit against the plaintiffs and their lawyers, including Steven Donziger and Pablo Fajardo, contending the judgment was obtained via fraud.

The declaration Monday by Guerra was filed by Chevron in support of that lawsuit.

According to the document, the Chevron case had been assigned to Judge Nicolas Zambrano in 2009. Guerra said Zambrano at times ask him to ghostwrite civil rulings for him for $1,000 a month, which he says was illegal.

The case was then given to another judge, Juan Nunez, who later had to recuse himself due to allegations of bribery brought by Chevron after Nunez was caught on tape discussing the case. Zambrano then took charge again.

In his sworn declaration, Guerra wrote: "Zambrano told me he was in direct contact with Mr. Fajardo and that the Plaintiffs' representatives had agreed to pay him USD $500,000 from whatever money they were to collect from the judgment, in exchange for allowing them to write the judgment in the Plaintiffs' favor."

Guerra in his declaration said he received $38,000 from the company for the costs of providing the evidence. Kent Robertson, a spokesman for Chevron, confirmed Chevron has agreed to pay Guerra's family $10,000 for monthly living expenses and $2,000 for housing.

"He's a scoundrel," Fajardo told Reuters on Monday. "I've never met Mr. Guerra, but I know who he is and how he's being paid by Chevron."

In a statement from the Ecuador plaintiffs last week, Fajardo said Guerra had at one point offered to provide testimony to the rainforest communities if they would pay him, though they had refused.

Zambrano could also not be contacted on Monday. He has been out of the public eye since his dismissal last February for releasing a drug trafficker in an act described by the judicial watchdog as "obvious negligence."

The New York case is Chevron Corporation v. Donziger, et al., U.S. District Court, Southern District of New York, 11-00691.

(Reporting by Nate Raymond in New York, Braden Reddall in San Francisco, and Eduardo Garcia Gil and Alexandra Valencia in Quito; Editing by Lisa Shumaker)