Mrs Percy, 62, was charged with aggravated trespass after climbing over the fence at RAF Croughton, Northamptonshire, at the time of the invasion of Iraq in March. Despite its name, the base is a US-controlled satellite communications centre.

Mrs Percy was carrying a Stars and Stripes on which she had written "War on Iraq ... Immoral ... Unlawful ... Madness". Last month she draped the same flag on the railings of Buckingham Palace during President Bush's state visit, an incident over which she was not charged.

Mrs Percy said that in the Croughton grounds she was stopped and searched by American security personnel who had no authority to do so.

With her lawyer, Raza Husain, she also argued she could not be charged with trespass because what was going on at Croughton was unlawful. The defence argued that Croughton was playing a crucial role helping the US pursue the war against Iraq.

The first trial was abandoned after the district judge at Northampton magistrates court stood down when he admitted being party to discussion with the US authorities in which they said they could not reveal documents setting out security procedures at the base.

Mrs Percy had argued that proceedings in an English court were in effect being conducted by US officials. The prosecution said the documents were not relevant, even though it had not seen them.

At the second trial this week, the US authorities insisted it was up to them, not the English court, to decide what should be released.

In the end, irrespective of the documents, Mrs Percy lost the case because the district judge, Richard Holland, ruled that in law there was no distinction between arguments about the legality of war - an issue which can be argued before the courts - and arguments about how a war should be conducted - an issue which the courts have said they cannot rule on.

He said Mrs Percy had "placed the flag in a secure area at a time of war and was capable of being an aggravating feature". Disrupting the Croughton base could have threatened "national security".