The women were contesting an order by North Yorkshire County Council to clear the site, in a lay-by near the base, in a legal action that has been going on for more than 18 months.
But Mr Justice Hooper, sitting in Leeds, ruled that the caravan came under the same highway regulations as a nearby mobile cafe.
The caravan was the last permanent peace camp in the country, born out of a tradition going back to Greenham Common in the early 1980s. It has been in place since 1993, but a further nine caravans were removed in November last year after earlier court rulings.
Protest leaders had previously pledged to remain on the site whatever the court ruled, even to the point of breaking the law. One woman involved in a separate protest at the base was imprisoned earlier this year for defying a court order banning her from the site.
The Menwith Hill Women's Peace Camp claims that the council was pressured into taking action against it by the Ministry of Defence, which was worried about the attention the camp was drawing to the base's activities. Documents released during the case confirm this, the protesters say.
Last year the women caused a row in the European Parliament over claims that Menwith Hill, run by the American National Security Agency on a lease from the Ministry of Defence, was dealing in commercial espionage as well as military monitoring. Security experts in Germany estimate that their country loses at least pounds 7bn a year in industrial secrets stolen as a result of commercial espionage.
Helen John, 61, who moved to the area to set up the protest camp, said that she was pleased but surprised by the court ruling. "The fact that we have been given a right to remain and protest means that we have the right to ask those in elected positions to go a lot further forward in trying to find out what is actually happening within that base," she said.
Menwith Hill already has 27 "golf ball" surveillance units, and a further two are due to be built in the near future. It has been claimed that the expansion will be part of the United States' Space Based Infra Red System, a version of "Star Wars" technology which would also have the ability to target satellites in space. This has been denied by the Ministry of Defence.
This capacity, together with the base's role in nuclear capability and a worldwide surveillance network, are the targets of the protesters. In modern warfare it is women and children who suffer disproportionately, they say.
North Yorkshire County Council yesterday welcomed the judge's decision on the grounds that it meant that the current, permanent caravan would have to be removed.
Alan Burns, the council's maintenance manager, said: "It is a balanced and sensible judgment which not only recognises the county council's role as highways authority, but also the right of protest - something which we have never tried to stop."Reuse content