WASHINGTON (Reuters) - Chief Justice John Roberts expressed confidence on Saturday in the decisions by his Supreme Court colleagues on when to recuse themselves, an issue that has emerged in the legal battle over President Barack Obama's healthcare overhaul law.

Some Democrats in Congress have called for Justice Clarence Thomas to be recused because of his wife's work for conservative groups that opposed the law while some Republicans have called for Justice Elena Kagan's recusal because of her prior position in the Obama administration.

Neither justice has removed themselves from the healthcare cases, which have been scheduled for arguments in late March, with a decision expected by the end of June.

Roberts in his annual year-end report on the federal judiciary did not mention by name the healthcare cases, but said he has complete confidence in the ability of his colleagues to decide when recusal was warranted.

In recusal questions, he said longstanding judicial ethics guidelines state a judge "should not be swayed by partisan demands, public clamor or considerations of personal popularity or notoriety, nor be apprehensive of unjust criticism."

Roberts said his colleagues were jurists of "exceptional integrity and experience" whose character and fitness have been examined during a rigorous appointment and confirmation process.

"We are all deeply committed to the common interest in preserving the court's vital role as an impartial tribunal governed by the rule of law," he said.

Some Republicans in Congress have sought documents about Kagan's role during the discussions about legal challenges to the healthcare law when she was Obama's solicitor general before she joined the court.

She testified during her confirmation hearing in 2010 that she "attended at least one meeting where the existence of the litigation was briefly mentioned, but none where any substantive discussion of the litigation occurred."

None of the parties in the healthcare cases has sought the recusal of the justices.

The Obama administration is defending the law as constitutional while 26 states and an independent business group are challenging the law and urging that it be struck down.

Roberts said each individual justice must decide whether recusal was warranted in a particular case.

Because the Supreme Court consists of nine members who always sit together, if a justice withdraws from a case, the court then would sit without its full membership, he said.

A recusal raises the prospect of a 4-4 split that would not decide the merits of a dispute.

"A justice accordingly cannot withdraw from a case as a matter of convenience or simply to avoid controversy," Roberts said.

(Reporting By James Vicini; Editing by Vicki Allen)

Mochila insert follows.