The New York attorney general's office has ordered Donald Trump's charitable foundation to immediately stop fundraising in the state, saying it is not registered to do so.
The head of the attorney general's Charities Bureau, James Sheehan, wrote in a letter to a lawyer for the Republican presidential nominee's foundation that failure to stop immediately and answer demands for all delinquent financial reports within 15 days "shall be deemed a continuing fraud upon the people of the state of New York".
The letter was dated and sent on Friday.
Democratic attorney general Eric Schneiderman has been investigating the Donald J Trump Foundation after Washington Post reports that foundation spending personally benefited the presidential candidate.
The newspaper, citing tax records, also reported that the charity has been funded entirely from outside donations since 2008, when the candidate made his last contribution to it.
The attorney general's office said the Trump Foundation had a registration for an organisation with assets in New York but the law requires a different registration for those that solicit more than 25,000 dollars (£20,000) a year from the public.
"Based on information received by the Charities Bureau to date, the Trump Foundation was engaged in solicitation or fundraising activities in New York State in 2016 and was not registered with the Charities Bureau pursuant to Article 7-A, and thus was not permitted to engage in such activity during this period," Mr Sheehan wrote.
The Trump campaign has said the foundation intends to co-operate with the investigation. The campaign previously called Mr Schneiderman "a partisan hack who has turned a blind eye to the Clinton Foundation for years and has endorsed Hillary Clinton for president".
"While we remain very concerned about the political motives behind AG Schneiderman's investigation, the Trump Foundation nevertheless intends to co-operate fully with the investigation," Trump campaign spokeswoman Hope Hicks said.
In September interviews, Mr Schneiderman told Politico that in June his office had communicated with foundation lawyers about what appeared to be an unlawful campaign contribution and did not make a public fuss about it and hold a press conference.
"If I'm a traffic cop, but I'm a Democrat, and (Trump) speeds by me, I have to give him a ticket. It's that simple. Charities have to follow the rules," Mr Schneiderman said.
His spokesman Eric Soufer said: "The attorney general's office is the sole regulator of charities in New York State, and when evidence of clear misconduct is brought to our attention, we take action."