The letter, spearheaded by the organisation Lawyers Defending American Democracy, called for immediate suspension of the licence pending an investigation of Giuliani, who served as a federal prosecutor and a New York City mayor before Trump hired him as to be his personal lawyer.

"This complaint is about law, not politics," said the letter to the attorney grievance committee of the Appellate Division of the Supreme Court of the State of New York, First Judicial Department. It was signed by more than 40 lawyers.

Giuliani did not immediately respond to requests for comment.

The grievance committee, part of New York state's judiciary, has the power to censure and suspend lawyers and revoke their law licences.

Giuliani led the legal team that tried to overturn Trump's election defeat but failed to produce any evidence of widespread fraud. The team lost dozens of court cases in battleground states and at the US Supreme Court, and President Joe Biden's victory was confirmed.

Criticisms of Giuliani escalated after the deadly January 6 attack on the Capitol by a pro-Trump mob. Shortly before, Trump told supporters to "fight like hell" and Giuliani urged "trial by combat".

The letter to the disciplinary body cited Giuliani's false statements in court and in public about widespread voter fraud, as well as his call for "trial by combat".

Lawyer suspensions and disbarments are very unusual, said Brian Faughnan, a lawyer in Tennessee who defends other lawyers in ethics matters.

But the letter laid out strong evidence against Giuliani and he may face a public reprimand, Faughnan said.

"If I got a complaint like that on behalf of a client, I would be worried," he said.

Australian Associated Press