The challenge, which has been brought by a single father, has infuriated the whole of the United States Senate, President George Bush and the district court for eastern California.

But Michael Newdow has defied the criticism and will argue today that the phrase "one nation under God, indivisible, with liberty and justice for all," recited by American schoolchildren across the country, violates the separation of church and state.

With one of the most conservative judges removed from the case, after he publicly criticised the challenge before it appeared before him, Dr Newdow has a reasonable chance of success.

Children in public schools in nine western US states have already been barred from reciting the full pledge.

Dr Newdow, 50, filed the legal challenge four years ago on behalf of himself and his nine-year-old daughter, based on his status as a parent of a schoolchild who was forced to say the pledge every day.

The judge dismissed it in two paragraphs and Dr Newdow, who is a passionate atheist, appealed.

After legal wrangling, Dr Newdow was granted the right to present the case before a three-judge panel, which shocked the country when it ruled 2-1 in his favour in June 2002.

In February the court refused to reconsider its ruling, prompting the attorney general, John Ashcroft, to ask the supreme court to review the case. A few days later the US Senate voted in support of keeping the phrase "under God" by 99-0. President Bush has described the ruling as "ridiculous".

The pledge of allegiance was first written - without the phrase "under God" - in 1892 in Boston, and recited by schoolchildren, a tradition that continues today.

The pledge was endorsed by Congress in 1942, just after the US joined the second world war.

In 1954, at the height of McCarthyism, "under God" was added to distinguish the "inalienable" rights of US democracy from those of communist states, where rights were conferred on citizens.

"We will defend the ability of Americans to declare their patriotism through the time-honoured tradition of voluntarily reciting the pledge," said Mr Ashcroft.

One complication for Mr Newdow is his relationship with his former partner and the mother of the child, Sandra Banning, 44.

She says that their daughter, who cannot be named for legal reasons, is a regular church-goer who likes to lead her class in reciting the pledge and should not be included in the suit.

If the supreme court judges decide that his daughter's name should not be on the suit, the case could be dismissed.