Republican presidential candidate Donald Trump speaks April 18 during a campaign stop in Buffalo, N.Y. (John Minchillo/AP)

Donald Trump's presidential campaign last month warned a prominent pro-Trump super PAC — which has put at least $1 million into advertising for the billionaire — that it is not authorized to fundraise on the candidate's behalf in a formal letter sent by the campaign's legal counsel.

The letter to the Great America PAC, signed by attorney Donald McGahn of Jones Day, stressed that the campaign believes the organization's efforts run counter to the anti-big money message Trump has emphasized on the campaign trail. It said that the PAC is "using the name, image and slogan of Donald J. Trump in connection with your fundraising activity in a way that is likely to cause confusion."

"In other words, potential supporters could be easily confused that when they make a contribution to your organization, they are supporting Donald J. Trump for President's campaign," wrote McGahn.

The letter, reported and published by Bloomberg BNA on Friday, was sent on Jones Day letterhead on March 23.

Trump has made self-funding a key message in his White House bid, and supporters around the country have flocked to the billionaire candidate because they believe his personal wealth shields him from being influenced by special interests. "Mr. Trump is self funding his campaign and has disavowed all Super PACs," campaign spokesperson Hope Hicks said in a statement to The Post on Friday when asked about the group's continued activity.

But that message risks being derailed by super PACs that have sprung up to support the real estate mogul’s presidential run. The letter from McGahn signaled that Trump remains concerned about complicating his anti-special interest messaging.

The PAC has drawn significant attention, particularly after veteran GOP strategist Jesse Benton — a longtime ally to Senate Majority Leader Mitch McConnell and Sen. Rand Paul — joined the group last month as its chief strategist. The PAC has pledged to put $1 million into TV ads and last month said it aims to raise at least $15 million, according to a report by the Wall Street Journal.

The PAC, which is chaired by former Tea Party Express chairwoman Amy Kremer, was founded earlier this year as TrumPAC. But the committee was forced to change its name after the official Trump campaign reached out to disavow the misuse of Trump's image and brand. The campaign has now requested that the Great America PAC "refund any funds raised" and to make the distinction between the PAC and the campaign more explicit.

"Great America PAC is bringing the grassroots and high dollar donor communities together," said Benton said in an email Friday. "We are continuing to build a robust and effective finance organization that Mr. Trump will need after he secures the nomination."

"Nice letterhead," said Dan Backer, treasurer of Great America PAC, when asked about the letter Friday in an email.

The Trump campaign has had an uneasy relationship with super PACs in the past. Last year, The Washington Post reported on professional ties between the Make America Great Again super PAC and the official political operation. The super PAC shut down and the campaign issued a statement disavowing all PACs not directly affiliated with the official campaign.

The campaign has not, however, ruled out using super PACs to fund a general election fight, which could be exorbitantly expensive.

"Mr. Trump is personally funding his own campaign in connection with the primaries with his vast personal fortune," reads the carefully worded letter.