Barack Obama today nominated a lawyer with no previous judicial experience to the US supreme court, risking a political showdown with the Republicans but potentially meaning that the nine-member court has three women for the first time.

Introducing Elena Kagan, 50, in the White House's East Room, Obama described her as a friend, and said she would show independence and integrity.

Kagan, who would also be the youngest justice, is the president's solicitor general, and now the government's advocate before the bench. She was the first woman dean of Harvard Law School.

The US Senate, where there is a Democratic majority, now has to approve the appointment of Kagan, who stands to be the first court member in nearly 40 years who has not been a judge.

The Republicans could try to stall the nomination of a candidate who served as an associate counsel in Bill Clinton's White House. When she became the first female solicitor general last year, only seven Republicans supported her.

The lack of time on a judicial bench could give critics an opening to question her qualifications for a seat at the court but the absence of politically controversial rulings in previous judgements could also deny them ammunition.

Her support of centralised government power as well as her reaching out to conservatives while head of the law school have disturbed liberals too.

She annoyed the right when she briefly barred military recruitment on campus because the ban on openly gay people joining the armed forces went against school anti-discrimination policy.

Kagan would replace retiring justice John Paul Stevens and might help counter what has been seen as a swing to the conservatives on the court, which Obama recently criticised in his State of the Union speech.

He claimed one of its rulings could "open the floodgates" for special interests, including foreign companies, to exert more influence on election campaigns, and is known to believe that the justices, who hold a pivotal role in the US constitution, do not give the average citizen sufficient voice.

Last year, Obama successfully nominated Sonia Sotomayor to be the first Hispanic member of the supreme court.