Judges on Friday overturned the decision by York Crown Court, which said the convictions against retired midwife Helen John and retired physics teacher Anne Lee could not stand because the bylaws to keep members of the public away from RAF Menwith Hill, in Yorkshire, were invalid.

But Lord Justice Buxton and Mr Justice Collins ruled the Crown Court had got the law wrong in September 1997 and the convictions must be reinstated.

Ordering the Crown Court to dismiss the women's appeals, Lord Justice Buxton said the bylaws had been declared void by York Crown Court because half the Ministry of Defence-owned land was occupied by sheep and not used for military purposes.

But the judge said the bylaws were valid because they applied to land "appropriated for military use".

'Fight continues'

Responding to the court's decision, Ms John, 61, said she was ready to break the bylaws again in protest at US Government eavesdropping operations at Menwith Hill, near Harrogate, North Yorkshire.

"I will soon break the bylaws again.

"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 but is a vital part of American military and economic espionage.

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

Ms John, who lives at a women's peace camp on the A59 near what is known as "Womenwith Hill", said a decision must be made on whether to take court action against 100 other campaigners who had broken the bylaws.

Pressure group Liberty recently alleged the listening post, reputed to be the largest overseas US spy base, is used by British intelligence agencies and police to avoid having to obtain Home Office warrants authorising telephone-tappings.