Squire Patton Boggs has ended its formal working relationship with US President Donald Trump's personal lawyer Michael Cohen, following an FBI raid on Cohen's offices in New York.

"The firm's arrangement with Mr Cohen reached its conclusion, mutually and in accordance with the terms of the agreement," the firm said in a statement. "We have been in contact with federal authorities regarding their execution of a warrant relating to Mr Cohen. These activities do not relate to the firm and we are in full cooperation."

Cohen, a longstanding personal lawyer to the president, formed a strategic alliance with Squire Patton Boggs in April last year. Details of the business partnership between were sparse; however, the US firm announced that Cohen would help "advance the interests" of the firm's clients and work with its notable lineup of lobbyists.

As recently as 15 February, the firm indicated to Legal Week sister title The National Law Journal that its strategic alliance with Cohen was still in place. As of Monday afternoon, the firm's press release trumpeting the partnership last year was no longer visible on the firm's website. The firm did not answer questions about exactly when its alliance with Cohen finally ended, or whether Monday's FBI raid took place at the firm's offices.

FBI officials reportedly swarmed Cohen's offices at Rockefeller Center in midtown Manhattan, as well as a hotel where Cohen was staying. Squire Patton Boggs – which is also currently advising Cambridge Analytica on the investigation into its use of Facebook data for the Trump election campaign – is one of several law firms with offices at Rockefeller Center.

According to the statement by Cohen's own lawyer, McDermott Will & Emery partner Stephen Ryan, the FBI seized privileged communications between Cohen and "his clients" on behalf of the US Attorney's Office for the Southern District of New York, Ryan said. The move was taken, in part, through a referral by the Office of Special Counsel, Robert Mueller, according to Ryan.

Ryan said the move by the office of US Attorney Geoffrey Berman was "completely inappropriate and unnecessary".

"It resulted in the unnecessary seizure of protected attorney-client communications between a lawyer and his clients," Ryan said. "These government tactics are also wrong because Mr Cohen has cooperated completely with all government entities, including providing thousands of non-privileged documents to the Congress and sitting for depositions under oath."

Cohen long served as Trump's personal lawyer ahead of his election as president in 2016. Most recently, he has found himself at the centre of controversy over a $130,000 payment he said he made on his own to adult film actress Stephanie Clifford, better known as Stormy Daniels, just before the election.

A spokesman for the Manhattan US Attorney's Office declined to comment. An attempt to reach Cohen by phone and email was unsuccessful.

Berman was appointed by US Attorney General Jeff Sessions in January, pending a nomination by Trump for confirmation by the US Senate. That nomination has yet to be made. US Senator Kirsten Gillibrand of New York has vowed to block Berman's nomination over reports Trump personally interviewed Berman, then a shareholder at Greenberg Traurig, ahead of his appointment.

The privileged communications seized by prosecutors will require a special process for federal agents and prosecutors to be able to use them, according to former prosecutors with experience handling communications usually protected by attorney-client privilege.

A firewall is erected between the 'dirty' side of the investigation – agents and prosecutors who conduct a filter review of the material that may include information the government should not be privy to. Any communications authorities want to be able to use requires the petitioning of a court for a crime fraud exception. The communications would have to show the lawyer was perpetuating or facilitating a criminal act.

Once the judge involved in the case grants the petition by the 'dirty' side of the investigation, the relevant information that has been cleared is handed off to the 'clean' side, which proceeds with the investigation.

This would not be the first time federal investigators sought a crime fraud exception to the attorney-client privilege. Recently, US District Chief Judge Beryl Howell of the District of Columbia granted the government's request to compel an attorney connected to Trump's former campaign manager Paul Manafort to testify before a grand jury in that prosecution.