Donald Trump's charitable foundation has been ordered by New York's attorney general to stop fundraising.
Prosecutors issued a "notice of violation" after determining the foundation had no proper registration.
The notice directs the Trump foundation to "immediately cease soliciting contributions or engaging in other fundraising activities in New York".
Meanwhile, Mr Trump defended his tax practices after a report found he had not paid taxes in nearly two decades.
The New York Times disclosed a portion of Mr Trump's 1995 tax returns on Sunday, showing that he declared losses approaching $1bn that year, which would have allowed him to legally avoid paying income tax for as many as 18 years.
The Republican presidential nominee has not denied the bombshell report, but instead told supporters in Colorado that he "brilliantly" navigated the complicated US tax code.
"I understand the tax laws better than almost anyone," he said, adding that as a real estate developer and businessman, he had "legally used the tax laws to my benefit and and to the benefit of my company".
He reiterated his stance that his business acumen makes him uniquely qualified to reform the tax code.
Mr Trump declined to comment on his foundation's alleged violations.
James Sheehan, head of the attorney general's charities bureau, sent the letter to the foundation on Friday, according to a copy provided by press office of state Attorney General Eric Schneiderman.
The foundation had neglected to register under article 7A of New York's Executive Law, which is required for any charity soliciting more than $25,000 (£19,440) a year.
The foundation has relied on donations from others since 2008, records show.
Hope Hicks, Mr Trump's spokeswoman, said in a statement that the foundation would co-operate fully.
When Mr Schneiderman's office revealed last month that it was investigating the foundation, the Trump campaign called him "a partisan hack."
The Democratic prosecutor has endorsed Mr Trump's opponent, Hillary Clinton, for president.
Mrs Clinton used the latest revelations to lambast her opponent on Monday, telling supporters in Toledo, Ohio: "While millions of American families - including mine and yours - were working hard paying our fair share, it seems he was contributing nothing.
"Trump represents the same rigged system that he claims he's going to change."
Mr Schneiderman has been looking into the foundation following Washington Post reports that the charity's spending personally benefited the candidate.
Friday's letter directed the foundation to provide the legal documents required of charities that solicit donations from the public, including audited financial statements and annual financial reports, within 15 days.
"The failure immediately to discontinue solicitation and to file information and reports" with the charities bureaus "shall be deemed a continuing fraud upon the people of the state of New York", Mr Sheehan wrote.
Mr Schneiderman's office has reportedly been investigating the Donald J Trump Foundation since at least June when it formally questioned a 2013 donation to a group backing Republican Florida Attorney General Pam Bondi.
The $25,000 payment came at a time when Mrs Bondi's office was reportedly considering whether to open a fraud investigation into Trump University.
The fraud investigation never happened, though Mrs Bondi denies the decision was influenced by the donation she received.
The Washington Post also reported this month that Mr Trump used more than $250,000 of foundation money to settle lawsuits that involved his businesses.