"At a time when the most extremist ideas are gaining ground, and when growing numbers of reporters are being killed or taken hostage, arresting a journalist in a democratic country is more than a crime: it's a miscarriage of justice," the group said. Its 27 members include the Spanish film director, Pedro Almódovar, the German literary Nobel laureate, Gunther Grass, and the former chief BBC reporter, Kate Adie.
Ms Miller was sent to a federal prison in the Virginia suburbs outside Washington DC on 6 July for refusing to reveal her sources in the case of Ms Plame, whose identity as a CIA agent was revealed to American journalists in 2003, apparently in violation of federal law.
No working member of the US press in recent history has spent more time behind bars than Ms Miller. She has already overtaken William Farr, a reporter for the Los Angeles Herald-Examiner, who was jailed in 1972 for 46 days for not revealing sources in a criminal case.
Democrats said Ms Plame's name was leaked by the White House in retaliation against her husband, ambassador Joseph Wilson, who wrote an article in The New York Times accusing the Bush team of overstating the evidence regarding Saddam Hussein's possession of weapons of mass destruction.
After a special prosecutor was appointed to investigate the leak, Ms Miller and other US journalists were required to testify to a grand jury about what they had been told and by whom. Ms Miller and Matthew Cooper of Time magazine refused. The latter finally agreed.
Friends and supporters say Ms Miller remains certain she made the right decision to stand up for the right of reporters to protect their sources. So she is likely to stay in jail at least until the grand jury completes its work on the case, which will probably be some time in October.
Mr Cooper said Karl Rove, the senior adviser to President George Bush, told him about Ms Plame and her job. Mr Rove had told the grand jury he had had no such knowledge.
The furore - which triggered calls from many Democrats for President Bush to fire Mr Rove - has subsided over recent weeks. Even the plight of Ms Miller seems to have been briefly forgotten by the wider press.
But that may quickly change when Mr Bush returns to Washington from his summer break in Texas. The special prosecutor, US Attorney Patrick Fitzgerald, could still issue criminal charges in September or October.
If any are directed at members of the White House, the scandal could quickly turn damaging for President Bush. Mr Fitzgerald has already extended his investigation beyond its original remit, to discover who leaked Ms Plame's name and whether laws were broken. He is now also looking into whether officials at the White House lied to the grand jury and tried to cover up what was said to reporters.
Ms Miller has supporters and critics. Last week, the former presidential candidate and Republican Bob Dole lamented her incarceration and voiced support for a new federal law protecting reporters from legal consequence for protecting sources.
Sympathy for Ms Miller has been tempered in other quarters because of news articles she wrote before the Iraq war supporting the administration's claims that Iraq had weapons of mass destruction.Reuse content