NEW YORK (REUTERS) - A longtime senior executive at Mr Donald Trump's family business pleaded guilty on Thursday (Aug 19) to helping the company engineer a 15-year tax fraud, in an agreement that will require him to testify about its business practices at an upcoming trial.

Allen Weisselberg, 75, the former chief financial officer (CFO) at the Trump Organisation, entered his plea to all 15 charges he faced in a New York state court in Manhattan.

Weisselberg, who has worked for Mr Trump for about a half-century, is not expected to cooperate with Manhattan prosecutors in a larger probe they are conducting into Mr Trump.

But his plea will likely strengthen their case against the former president's company, which manages golf clubs, hotels and other real estate around the world.

"This plea agreement directly implicates the Trump Organisation in a wide range of criminal activity and requires Weisselberg to provide invaluable testimony in the upcoming trial," Manhattan District Attorney Alvin Bragg said in a statement. "We look forward to proving our case in court."

In a statement, a Trump Organisation spokesman called Weisselberg a "fine and honourable man" who has been "harassed, persecuted and threatened by law enforcement, particularly the Manhattan district attorney, in their never ending, politically motivated quest to get President Trump".

She also said the company will not admit guilt because it did nothing wrong, and looked forward to its day in court.

Jury selection for the trial is scheduled to begin on Oct 24, 15 days before the Nov 8 midterm elections, where Trump's Republican Party hopes to recapture both houses of Congress from Democrats.

Mr Trump, who is embroiled in a series of other legal fights, has yet to say whether he plans another White House run in 2024.

The Trump Organisation has pleaded not guilty, and faces possible fines and other penalties if convicted. Mr Trump has not been charged or accused of wrongdoing in the case.

Five months at Rikers

Prosecutors charged the Trump Organisation and Weisselberg in July 2021 with scheming to defraud, tax fraud and falsifying business records for awarding "off-the-books" perks to senior executives.

Weisselberg was accused of concealing and avoiding taxes on US$1.76 million (S$2.44 million) of income.

This included rent for a Manhattan apartment, lease payments for two Mercedes-Benz vehicles, and tuition for relatives, with Mr Trump signing the tuition checks.

Weisselberg's plea agreement calls for him to serve five months at the Rikers Island jail, though he could be freed after 100 days. It also include five years of probation, and the payment of US$1.99 million in taxes, penalties and interest.

The jail sentence would begin after the Trump Organisation's trial concludes. Weisselberg could have faced 15 years in prison if convicted at trial, including on a grand larceny charge.

During Thursday's hearing, Weisselberg removed his mask as Justice Juan Merchan described each count of the indictment.

Weisselberg agreed that the accusations against him were true.

Weisselberg gave up the CFO job after being indicted, but remains on Mr Trump's payroll as a senior adviser.

Mr Nicholas Gravante, a lawyer for Weisselberg, said in a statement: "In one of the most difficult decisions of his life, Mr Weisselberg decided to enter a plea of guilty today to put an end to this case and the years-long legal and personal nightmares it has caused for him and his family."

Last Friday, Mr Merchan denied defence motions to dismiss the indictment, rejecting arguments that the defendants had been "selectively prosecuted" and that Weisselberg was targeted because he would not turn on his longtime boss.

Probe 'ongoing'

The indictment arose from an investigation by former Manhattan District Attorney Cyrus Vance, but lost steam after Mr Bragg became district attorney in January.

Two prosecutors who had been leading the investigation resigned in February, with one saying felony charges should be brought against Mr Trump, but that Mr Bragg indicated he had doubts.

In his statement on Thursday, Mr Bragg said the investigation remained ongoing.

Mr Trump has a series of pending legal battles.

Last week, FBI agents searched his home for classified and other documents from his time in office.

Two days later, Mr Trump was deposed in New York Attorney General Letitia James' civil probe into whether he misled banks and tax authorities about his business's assets.

He repeatedly refused to answer questions, citing his Fifth Amendment US Constitutional right against self-incrimination.

Ms James is assisting Mr Bragg in his criminal probe.

"Let this guilty plea send a loud and clear message: we will crack down on anyone who steals from the public for personal gain," Ms James said in a statement.