Magistrates jailed Lindis Percy, 64, after she failed to pay fines and court costs relating to a protest outside the United States Signal Intelligence Station, in Harrogate, North Yorkshire.

Percy had expected to be back at home on Friday night after an appearance at Harrogate magistrates' court where she argued, on a point of principle, that she would not pay the fine and £150 costs. But she was handed a seven-day sentence and sent to Low Newton jail in Co Durham.

Details of the peace campaigner's imprisonment emerged yesterday as the Prime Minister was forced to admit Britain's jails were "full to bursting point".

Her husband, Christopher Percy, said yesterday: "With the warnings about prison overcrowding, it was unexpected and extraordinary." Percy appeared in court with her right arm in plaster after breaking it while trying to evade arrest at the base.

The pensioner undertakes weekly vigils at the US intelligence station. She formed the Campaign for the Accountability of American Bases (CAAB) in 1992 and was sent to jail in March 2003, as the invasion of Iraq began.

Though her protests have resulted in 12 prison sentences before, including a four-and-a-half-month spell in Holloway jail, Percy was adamant that she was not guilty of the crime that led to Friday's court appearance: wilful obstruction of the highway outside Menwith Hill, on 10 and 31 January last year.

She was convicted after a three-day trial before Deputy District Judge Richard Manning in September. The judge dismissed her claims that she had been neither on the highway nor obstructing cars leaving the base at the time of the offences.

Andrea Seddon, the chairman of Harrogate magistrates' bench, said on Friday: "You were given the opportunity to pay but there was wilful refusal and we order that the (previously) suspended sentence takes effect from today.'' Percy is expected to be released early this week.

The case has raised new questions about the judiciary's inclination to jail protesters who appear to offer no threat to the public. They also include the climate-change protester Irene Willis, 61, who was jailed for 21 days for refusing to pay a fine for demonstrating outside the US air base at Lakenheath.

Percy, a trained midwife and nurse, has a history of embarrassing the authorities. Previous protests, which have resulted in her being arrested more than 150 times, have included breaching Buckingham Palace security and climbing its gates in October 2003, ahead of a visit by George Bush. In June, she confronted the former president George Bush Snr as he addressed business leaders in Yorkshire.

Her imprisonment comes ahead of another case before Harrogate magistrates. The campaigners Helen John and Sylvia Boyes were arrested a year ago under a little-noticed clause in the Serious Organised Crime and Police Act for offences which carry a possible 12-month prison term.