Giuliani Former New York City mayor Rudy Giuliani and then-presidential candidate Donald Trump. (Photo: Jeff Siner)

Greenberg Traurig partner Rudy Giuliani is joining US President Donald Trump's legal team in the special counsel investigation, leading to another leave of absence from his firm.

"Mr Giuliani is taking a leave of absence, effective today, for an unspecified period of time to handle matters unrelated to the law firm or its clients," said Richard Rosenbaum, executive chairman of Greenberg Traurig, in a statement.

Several news reports on Thursday afternoon cited interviews, including with Giuliani himself, confirming his plans to join Trump's legal team as part of the investigation by special counsel Robert Mueller related to Russian interference in the 2016 US election.

Giuliani would join Jay Sekulow and former Hogan Lovells partner Ty Cobb in representing Trump. According to several media reports, Jane Serene Raskin and Marty Raskin, a couple who run a Florida-based law firm, would also be joining the Trump team.

Many lawyers at top US firms have declined offers to join the president's legal team, including prominent partners at Gibson Dunn & Crutcher and Winston & Strawn. Last month, former Akin Gump Strauss Hauer & Feld partner John Dowd resigned from his role on Trump's lineup.

In late 2016, Giuliani, a former federal prosecutor and New York City mayor, took a leave of absence to work on the Trump election campaign before returning to the firm.

Earlier that year he had left the firm he founded, Bracewell & Giuliani, to join Greenberg as global chair of its cybersecurity and crisis management practice. Bracewell subsequently rebranded to remove his name from the firm's masthead.

In a statement issued by Sekulow and appearing in news reports, Trump said "Rudy is great" and has been in his friend "for a long time and wants to get this matter quickly resolved for the good of the country".

According to Sekulow's statement, Giuliani said it is "an honour to be a part of such an important legal team, and I look forward to not only working with the President but with Jay, Ty, and their colleagues".

In a Washington Post interview, Giuliani said he has been speaking with Trump for weeks about joining.

Giuliani, a former US Attorney of the Southern District of New York from 1983 to 1989, has been law firm colleagues with another US attorney, Geoffrey Berman, who left Greenberg to become the top Manhattan federal prosecutor in January. Trump personally interviewed Berman for the role, according to media reports.

Less than two weeks ago, the US attorney's office in the Southern District raided the home and office of Michael Cohen, a longtime personal lawyer for Trump. Shortly after, it was revealed that Berman had recused himself from the Cohen investigation. Deputy US attorney Robert Khuzami, a former Kirkland & Ellis partner, is now in charge of the investigation, court documents show.