Rudy Giuliani announced Thursday that he has resigned from his job at a Manhattan law firm to focus solely on representing President Donald Trump in the special counsel's Russia investigation.

Giuliani, a former New York City mayor and U.S. attorney, said he had originally planned to take a short leave of absence from his law firm, Greenberg Traurig, when he joined Trump's outside legal team in April. But both Giuliani and the firm said Thursday that the leave is now permanent.

"In light of the pressing demands of the Mueller investigation, I believe it is in everyone's best interest that I make it a permanent resignation," Giuliani said in a statement. "This way, my sole concentration can be on this critically important matter for our country."

The decision was first reported by The New York Times.

According to a source with firsthand knowledge of the firm’s internal discussions, Giuliani was the main topic of conversation as Greenberg Traurig began its annual conference in Las Vegas on Thursday, much to the dismay of the firm's managing partners.

On Thursday, partners said they had finally had enough, having grown increasingly frustrated with how Giuliani has handled himself in the last couple of weeks — especially his “leave of absence” status from the firm, which allows him to collect the president’s legal bills personally, rather than through the firm. This has meant that Greenberg Traurig had to endure a reputation risk from Giuliani's work and press appearances without the benefit of payment.

Giuliani previously told NBC News in an interview last week that it has not yet been determined how Trump will pay him, and said that he has offered to volunteer his services.

In a phone interview Thursday, Giuliani said that when he took on the role as the president's lawyer, he initially thought it would be "part-time and quick," but realized that "you can't just jump in and jump out." He called his departure from the firm "a mutual decision."

Richard A. Rosenbaum, Greenberg Traurig's executive chairman, confirmed Giuliani's departure in a statement to NBC News, saying, "After recognizing that this work is all consuming and is lasting longer than initially anticipated, Rudy has determined it is best for him to resign from the firm." The resignation became effective on Wednesday, the statement said.

Giuliani has caused a stir in a series of recent interviews, particularly one with Fox News' Sean Hannity earlier this month in which he revealed that the president had reimbursed his personal attorney, Michael Cohen, for a $130,000 payment to porn star Stormy Daniels despite the president previously denying knowledge of the transaction.

Daniels has claimed to have had a sexual affair with Trump, something the White House and Cohen have denied, and that she was paid to keep quiet just days before the 2016 election. In February, Cohen had said he used his own money to pay Daniels, and that Trump had not reimbursed him.

Giuliani's revelation raised eyebrows because neither Cohen's original outlay to Daniels, nor Trump's repayment were reported to the Federal Elections Commission as contributions to a campaign that could have benefited from the performer's silence.

The lawyer attempted to clarify his remarks last week, insisting that Trump had only recently found out that he had reimbursed Cohen and saying the payment did not amount to a campaign finance violation because it "was made to resolve a personal and false allegation in order to protect the president's family."