The sentence on forgery charges against Mr Nour, 41, who was the most prominent challenger to President Hosni Mubarak in September's presidential elections, has been widely criticised by human rights groups and other opposition figures. It has also cast another shadow over Egypt's claim to be democratising after a spate of elections that it had been hoped would set an example to much of the Arab world but which were themselves marred by Government-backed violence.
Scott McClellan, the White House spokesman, said the US was "deeply troubled" by the sentence and added that it "calls upon the Egyptian government to act under the laws of Egypt in the spirit of its professed desire for increased political openness and dialogue within Egyptian society, and out of humanitarian concern, to release Mr Nour from detention".
An earlier statement from the White House had said that the sentence handed down to Mr Nour on Saturday "calls into question Egypt's commitment to democracy, freedom and the rule of law". There was no immediate or explicit US threat to withhold any of the $2bn (£1.2bn) of annual financial and military aid Egypt receives from the US.
Mr Nour's lawyers said he would appeal the sentence, which was imposed when the court found in favour of prosecution claims that he had boosted his appeal by forging 1,000 signatures supporting his application to form his new party, Ghad (Tomorrow), which also contested last month's legislative elections.
But his family and lawyers said the prosecution destroyed the original, genuine, petitions and then substituted forgeries. They pointed out that the law requires only 50 such signatures on a petition and that he had no motive for the forgery.
As he was sentenced, Mr Nour, a former lawyer, locked in a courtroom cage, and looking pale and gaunt after being on hunger strike for more than a week in protest at his prosecution, began to chant: "Down with Mubarak."
Mr Nour won about 8 per cent of the popular vote on a liberal platform in the presidential elections, coming second to President Mubarak, who secured 89 per cent. He and all other Ghad Party members of parliament lost their seats in the legislature during the elections in November and December and Mr Nour claimed that his own defeat had been rigged. The government has repeatedly stood by its insistence that the judiciary is independent.
Nagui El Ghatrifi, the deputy leader of the Ghad Party and a former diplomat, said: "By having him in prison, the opposition will be more severely felt and I don't think their reaction will be peaceful." The Ghad Party also pointed out that the judge, Abdel Salam Gomaa, had jailed another prominent, dissident, the sociologist Saadeddin Ibrahim, guilty in 2002 of damaging Egypt's reputation abroad. Mr Ibrahim, who was later acquitted and released, said: "The Mubarak regime could not stand him so they fabricated a case against him."