WASHINGTON — Former Rep. Jesse Jackson Jr. — who already has served his prison sentence for looting his campaign funds — has surfaced in another criminal case, with new court records showing he may have taken gifts and cash from a Chinese billionaire while still in Congress.

The new allegations against Jackson are surfacing as a side matter in the criminal case against Macau real estate mogul Ng Lap Seng, who was indicted in October 2015 for bribing a United Nations official.

Seng’s lawyers are fighting a move by federal prosecutors in the U.S. Attorney’s office in the Southern District of New York in Manhattan to bring Jackson into the case to help prove the charges against Seng.

The Chicago Sun-Times reported last month that the former Democratic congressman in 2015 was trying to enter into a cooperation deal with Southern District of New York prosecutors after telling them of “potential violations of law by him and others” in an unspecified case, according to a letter obtained by the newspaper.

The new court filing by Seng’s lawyers details prosecutors’ allegations that Seng paid Jackson both before and after Jackson was in Congress — and channeled money through his wife Sandi Jackson as part of a consulting agreement.

According to the May 1 filing, prosecutors were planning to introduce into evidence that “starting in or about 2009, the defendant [Seng] agreed to and did provide gifts and payments, including cash, a watch, theater tickets, hotel rooms, and meals, to a certain United States representative . . . in return for the representative agreeing to introduce the defendant to other United States officials and businesspeople in the United States.

“After the representative left Congress, the defendant agreed to and did continue to make payments to the representative, including by providing cash and directing payments to the representative’s wife, purportedly as part of a consultancy agreement.”

Seng’s lawyers then state “the ‘representative’ to whom the government refers is former congressman Jesse Jackson Jr.”

Seng’s lawyers wrote their filing after prosecutors on April 17 informed them in a letter that “the government may seek to introduce at trial” evidence of “uncharged crimes, wrongs, or acts committed or caused to be committed by defendant Ng Lap Seng . . . as proof of the defendant’s motive, opportunity, intent, preparation, plan, knowledge, identity, and/or absence of mistake or accident with respect to the charges.”

The Seng filing also recounts the criminal pasts of both Jesse Jackson and Sandi Jackson, who are now embroiled in a contentious divorce. Jackson left Congress in 2012 and pleaded guilty — along with his wife, a former Chicago alderman — to charges related to looting $750,000 from his campaign funds between 2005 and 2012.

“Jackson’s illegal conduct had nothing to do with Mr. Ng. However, Jackson found himself in dire financial straits after his conviction and repeatedly solicited money from Mr. Ng,” Seng’s filing states. “The jury may well infer that because Mr. Ng associated with a corrupt politician, he is likely to be corrupt as well.”

If the allegations are true, Jackson might have taken impermissible gifts while a member of Congress.

But Jackson attorney Brendan Hammer told the Sun-Times on Monday that Jackson “has not been charged with any crime and is not a party to the case pending in New York.

“There is no indication, nor should there be any implication, of any wrongdoing on Mr. Jackson’s part,” Hammer said.

Hammer also told the Sun-Times, “As I indicated previously, any requests for comment should be directed to the United States Attorney’s Office for the Southern District of New York. As this matter involves the prosecution of a third party in an active case, it would be inappropriate to comment any further at this particular time.”

Sandi Jackson couldn’t be reached for comment.

Chinese billionaire Ng Lap Seng, center, leaves federal court with his attorney Benjamin Brafman, right, after he was released on bail in connection with a U.N. bribery scheme in 2015. AP File Photo/Kathy Willens

The letter obtained by the Sun-Times last month shows Jackson  proposed to secretly record conversations as part of a federal investigation.

The Aug. 16, 2015, letter from Assistant U.S. Attorney Daniel C. Richenthal in Manhattan to another Jackson attorney, John Colette, shows Jackson proposed to secretly record conversations as part of the federal investigation.

Richenthal is one of the federal prosecutors on the Seng case.

The Seng filing does not say if Jackson was trying to make a deal over the alleged Seng payments or if he became an informant.

Sandi Jackson — asked by the Sun-Times last month if Jackson ever became an informant, or of she was familiar with the offer — had no comment.

The filing from Seng’s lawyers does not deny or confirm the allegations about the Jackson payments; rather, they state “the payments Mr. Ng. allegedly made to Jackson while he was in Congress are irrelevant.”

Jackson, the filing states, “found himself in dire financial straits after his conviction and repeatedly solicited money from Mr. Ng.”

The filing also argues “the payments after Jackson left Congress are even less relevant, if that is possible. Mr. Ng had nothing to gain from someone who was tainted with scandal and no longer held public office. Indeed, the fact that Mr. Ng made these payments to Jackson’s family at a time when Jackson was wholly disgraced and powerless demonstrates a different character trait: Mr. Ng’s propensity to use his wealth to help people who ask for it.”

Ng has been  a controversial figure for years, since he was a subject of many news stories in connection with President Bill Clinton-era campaign finance scandals in the mid-1990s related to foreign money flowing to the Democratic National Committee.

Ng Lap Seng Memo by cfusco1972 on Scribd