Mary Martin was 11 years old when her father taught her to box. She would come home from school scratched and bruised, her ears ringing with abuse from the playground. Mary Martin had the unhappy distinction of being the granddaughter of Britain's last convicted witch.

Mrs Martin knew her grandmother, Helen Duncan, as a comforting woman she could trust, the granny with a special gift: talking to spirits. But this was April 1944, at the height of the war with Germany. Mrs Duncan had just been branded by an Old Bailey jury as a witch and spy guilty of revealing wartime secrets.

Some 50 years after Mrs Duncan's death, a fresh campaign has been launched to clear her name, with a petition calling on the home secretary, John Reid, to grant a posthumous pardon. Her conviction, said Mrs Martin, was simply "ludicrous".

The appeal is winning international support from experts in perhaps the world's most infamous witch trial: the conviction and execution of 20 girls, men and women at Salem, Massachusetts, in 1692. "Helen Duncan was very much victimised by her times, and she too suffered," said Alison D'Amario, education director at the Salem Witch Museum.

Mrs Duncan, a Scotswoman who travelled the country holding seances, was one of Britain's best-known mediums, reputedly numbering Winston Churchill and George VI among her clients, when she was arrested in January 1944 by two naval officers at a seance in Portsmouth. The military authorities, secretly preparing for the D-day landings and then in a heightened state of paranoia, were alarmed by reports that she had disclosed - allegedly via contacts with the spirit world - the sinking of two British battleships long before they became public. The most serious disclosure came when she told the parents of a missing sailor that his ship, HMS Barham, had sunk. It was true, but news of the tragedy had been suppressed to preserve morale.

Desperate to silence the apparent leak of state secrets, the authorities charged Mrs Duncan with conspiracy, fraud, and with witchcraft under an act dating back to 1735 - the first such charge in over a century. At the trial, only the "black magic" allegations stuck, and she was jailed for nine months at Holloway women's prison in north London. Churchill, then prime minister, visited her in prison and denounced her conviction as "tomfoolery". In 1951, he repealed the 200-year-old act, but her conviction stood. Mrs Martin recalls that news of Mrs Duncan's conviction spread through her working-class suburb of Craigmillar in Edinburgh like a virus. "It was in all the papers, and of course the evil eye, witch-spawn - you name it, we were called it. My older sister, Helen, just wouldn't mention it. She shut it out of her mind. It was grim. I was only 11 years old, and children can be the cruellest under the sun. It taught us how to look after ourselves, I can tell you that much."

She remains nonplussed that the case ever went to court. "The arrest was silly really. If they'd spoken to her she would've stopped giving seances until the war was over. Let's be honest: she'd two sons in the navy, and one in the RAF, and my father in the army. So why would she turn around and put the country at risk?"

The petition has been set up by an arts festival and the holder of a medieval barony, Gordon Prestoungrange, in the coastal town of Prestonpans east of Edinburgh, a few miles from Mrs Martin's home. Two years ago, Dr Prestoungrange used his ancient powers as the local baron to pardon 81 women and men from the area executed for witchcraft in the 16th and 17th centuries. "The prosecution and conviction of Helen Duncan as a witch was clearly as much of an injustice as those of the 16th and 17th centuries," he said.

"It's hardly credible that a 20th century court would be prepared to convict someone of witchcraft - within living memory of many in this present government. As well as the deprivations suffered by Helen Duncan in prison, the effect of the stigma on her family was and remains considerable."

Mrs Martin and her supporters face a battle to convince the Home Office to act. But Tony Blair's apology for Britain's role in slavery, and the official pardon for more than 300 first world war servicemen convicted of cowardice, have reinvigorated the campaign. Convicted witches are being pardoned across the US. Mrs Duncan died in 1956, three months after being arrested again in a police raid on a seance in Nottingham. Paranormal investigators denounced her as a fraud who used cheesecloth, rubber gloves and egg whites to create the "ectoplasm" she claimed to produce.

Mrs Martin insists her grandmother was a genuine spiritualist, "an ordinary woman with a gift. I just want her name cleared. She was never given the chance to defend herself at the trial. It was such an injustice. While all this was happening, our troops were preparing for D-day. Why did they spend 10 days trying an old lady for witchcraft?"

The witchcraft laws

Witch hunts reached their peak in the UK in the 17th century, when the church viewed witches as devil-worshipping heretics. In 1604 James I issued a statute against witchcraft. Numerous trials followed, including those instigated by Matthew Hopkins, self-appointed witchfinder general, from 1644 to 1647.

Hopkins travelled the south-east seeking out witches, using torture to secure confessions and using methods such as swimming - throwing the accused into a river and judging them innocent if they sank - to determine guilt. He is thought to have executed 200-400 "witches". In Manningtree, Essex, alone, he accused 36 women, 19 of whom were executed; a further nine died in prison.

The accused were overwhelmingly female, often widows with no family to protect them. Some were herbalists or healers, practices opposed by church teachings, and some probably did practise dark arts, though most were innocent. The last execution for witchcraft in England was in 1684, when Alice Molland was hanged in Exeter. James I's statute was repealed in 1736 by George II. In Scotland, the church outlawed witchcraft in 1563 and 1,500 people were executed, the last, Janet Horne, in 1722.

Gerald Brousseau Gardner founded the modern Wicca movement in the 1940s, 11 years before the repeal of Britain's witchcraft laws. Followers revere nature, worship a goddess and practice ritual magic. In the 2001 census, 7,000 people listed Wicca as their religion.

Katy Heslop