York Crown Court had decided that the convictions against retired midwife Helen John and retired physics teacher Anne Lee could not stand because the bylaws, introduced to keep uninvited members of the public away from the heavily-guarded RAF Menwith Hill were invalid.

But yesterday Lord Justice Buxton, sitting at London's High Court with Mr Justice Collins, ruled that the Crown Court had got the law wrong and the convictions must be reinstated.

Ms John, 61, said outside court that she was ready to break the bylaws again in protest at the US Government's eavesdropping operations at Menwith Hill, reputed to be the biggest overseas American spy base.

Ms John, now living at a women's peace camp on the A59 near the base, known as "Womenwith Hill", said today's ruling meant that a decision would have to be made on whether to proceed with court action against about 100 other women campaigners who had also broken the bylaws.

"I will very soon break the bylaws again," she said, "They are one layer of added protection for the secrecy surrounding a US base that does not act in the best interests of the British people.

"The fact is they can intercept all of the telecommunications throughout the British Isles as well as western Europe."

The pressure group Liberty expressed fears that the top-secret listening post was being used by British intelligence agencies and police to avoid having to obtain Home Office warrants authorising telephone-tapping.

Sheep

Ms John said: "We have seen an enormous amount of interest in Europe in the activities at Menwith Hill, which is prepared to take it more seriously than the British courts appear to do."

Lord Justice Buxton said the appeal was only concerned with the legality of the Menwith Hill bylaws, introduced in February 1996 under the 1892 Military Lands Act.

Ms John and Ms Lee were found guilty at Claro Magistrates' Court in Harrogate of entering Ministry of Defence-owned land without permission, in February and June 1996 and failing to leave "as soon as practicable" when asked to do so by a police officer.

In September 1997 the Crown Court allowed their appeals , saying the bylaws were void because more than half of the MoD land was occupied by sheep and not used for military purposes.

But Lord Justice Buxton said: "The bylaws are not rendered void by the fact that part of the land was not being actively used for military or operational purposes."

• Yorkshire CND's Menwith Hill website ,
• The Adventures of Flossie Mintballs ,
• Women Protesting at the Gates of the Military .
• The Global Network against Weapons and Nuclear Power in Space .