Mr Obama's choice of the liberal Sotomayor, a 54-year-old judge on the 2nd US Circuit Court of Appeals in New York, was unlikely to change the ideological balance of the high court because Justice Souter, 69, was part of the panel's liberal wing.

The court, which decides controversial cases on abortion and the death penalty, has been closely divided on many contentious issues, with a five-member conservative majority and four dissenting liberal justices. Judge Sotomayor was expected to be a reliable liberal vote.

Conservatives quickly moved to criticize the choice, but political analysts said that, barring an unexpected scandal, there was little chance the nomination could be derailed.

Supreme Court justices serve for life but their nomination must be approved by the US Senate, where Mr Obama's fellow Democrats have a majority.

'I have decided to nominate an inspiring woman who I believe will make a great justice, Judge Sonia Sotomayor of the great state of New York,' Mr Obama said in a White House event announcing his decision.

Judge Sotomayor, a child of Puerto Rican parents, is most widely known for her decision as a trial judge in 1995 to bar Major League Baseball from using replacement players, ending a nearly year-long strike.

'Some say that Judge Sotomayor saved baseball,' Mr Obama told the White House crowd.

Based on her rulings as a trial judge and then a US appeals court judge in New York, Judge Sotomayor's views would in many respects be similar those of outgoing Justice Souter.

As an appeals court judge, she has often sided with plaintiffs in cases involving race, sex, age and disability discrimination and has ruled against businesses in cases on environmental law and securities litigation. The two business rulings were later reversed by the Supreme Court. -- REUTERS