Rudy Giuliani, a recent addition to President Donald Trump's legal team, announced Thursday that he permanently resigned from his law firm to give his "sole concentration" to special counsel Robert Mueller's Russia probe.

Giuliani's increased commitment to defending the president in Mueller's probe of links between the Trump's 2016 campaign and the Kremlin comes less than a month after the former New York City mayor took a leave of absence from his firm, Greenberg Traurig.

"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."

Greenberg's executive chairman, Richard Rosenbaum, said Giuliani permanently left the firm on Wednesday.

Shortly after his role on Trump's team was announced in late April, Giuliani told CNN that he would be able to wrap up his efforts on the Mueller investigation in as quickly as "a couple of weeks."

Giuliani's initial intent, Rosenbaum said in a statement Thursday, "was to play a limited role, for a short period of time, to address specific matters for President Trump." But he added, Giuliani now recognizes that the new task is "all consuming" and "lasting longer than initially anticipated."

In a series of television and print interviews last week, Giuliani made waves when he revealed that the president repaid his personal lawyer, Michael Cohen, for a $130,000 payment made to porn star Stormy Daniels as part of a nondisclosure deal barring her from discussing an alleged affair with Trump.

Trump had previously denied knowledge of the payment. Cohen told the New York Times in February that the money for the payment came from his own pocket, and that neither Trump's campaign nor the Trump Organization were involved.

In April, Cohen's office and residence were raided by federal agents, who reportedly seized materials including documents related to the Daniels payment. Cohen's lawyer, Stephen Ryan, said the raids were the result of a referral from Mueller's team, and were reportedly signed off on by Deputy Attorney General Rod Rosenstein, who gained oversight over the special counsel's probe following Attorney General Jeff Sessions' recusal.

Jay Sekulow, a lawyer on Trump's legal team, did not immediately respond to CNBC's request for comment on Giuliani's announcement.