Theresa Longworth may have scandalised 19th-century high society by daring to demand recognition as the legal spouse of a bed-hopping cad. But, as Chloë Schama explains, hers was a heroic struggle

• Edinburgh's Old Town in the 1800s

That moment, on 12 April, 1857, was to lead to betrayal, abandonment and bitter recrimination, for the couple's scandalous romance shocked Victorian society, leading to a decade of byzantine legal actions beside which Dickens's Jarndyce and Jarndyce in Bleak House pales into insignificance.

The ardent Theresa, the daughter of a Manchester silk manufacturer, had relentlessly pursued the son and heir of Viscount Avonmore ever since they had met by chance five years earlier aboard a ship from Boulogne to Dover. She was just 19, returning home from the convent in France where she'd been educated.

Besotted with this virtual stranger, she wrote him copious letters, following the Royal Artillery officer to the Crimean War, where she became a nurse even before Florence Nightingale's arrival.

Despite her rapturous love for Yelverton, Theresa was not at ease about their clandestine "marriage", since the dubious ceremony was witnessed by neither family nor friends, let alone a priest. Even her landlady, Mrs Mary Gamble, who did not venture into Theresa's quarters in the narrow St Vincent Street lodging house where she rented three rooms in order to be close to Yelverton's Leith barracks, was not in attendance. Nonetheless, according to Scottish common law, Yelverton's declaration constituted a marriage.

Such "irregular" marriages were not uncommon in Scotland, points out Chlo Schama in her new book about the pair's steamy intrigue, Wild Romance: The True Story of a Victorian Scandal. The scandal was to occupy more lurid tabloid column inches than the John Terry love triangle or, indeed, Cheryl Cole's current marital troubles, while the ensuing multiple trials in Scotland, Ireland and England inspired novelists, such as Wilkie Collins, and songwriters, who devoted ballads to the affair.

Were Theresa and Yelverton married in the eyes of the law?

"In the 1850s, a 'regular' marriage may have required the proclamation of banns before a clergyman, but an 'irregular' marriage could be forged by a mere declaration of vows between two people, requiring neither witnesses nor written documentation, according to a contemporary legal authority," notes Schama, whose father is Simon Schama, the historian and TV presenter. Her mother, Ginny, is a prominent geneticist.

Though she would later say that she believed in the Edinburgh marriage's legitimacy, Theresa claimed that her religious scruples prevented her from sleeping with Yelverton, who, being a heartless cad and a bounder to boot, claimed later that they were already having sex anyway. A classic case of "she said, he said".

A few days after the makeshift ceremony, Theresa, apparently alarmed by his insistent demands, left Edinburgh for Hull, seeking refuge with friends. She was miserable, "half married and not married".

"Oh, for those blessed days when I could trust you," she wrote to Yelverton, who had retreated to Belle Isle, his family's ancestral seat in Ireland. He told Theresa she was to keep quiet about their Scottish "marriage". She became ill before travelling to Ireland, where, on 11 August, reader, she married him.

The wedding took place in a Roman Catholic church, in Rostrevor, at the foot of the Mountains of Mourne. There were no witnesses, apart from the officiating parish priest. By the end of the month, they had parted ways again. Six weeks later, they were reunited in Edinburgh.

Theresa had moved into rooms at 31 Albany Street, although she missed her St Vincent Street apartment, describing it as "a little room, five storeys high, where we have been so happy".

The couple kept a low profile, but her closest friends assumed they were newlyweds and it was no secret to Mrs Stalker, the owner of the house, and her staff that they slept together at night. But they didn't remain there, presumably because Yelverton did not wish his friends to form the same impression. The couple set off on a 12-day tour of the Highlands, via Linlithgow, Dunblane and Dunfermline, leaving a trail of witnesses who would later testify that they lived together as man and wife, signing "Mr and Mrs Yelverton" in the Doune Castle visitors' book. They then made a continental trip together. On their return to Edinburgh, Yelverton promised, "I'll have a cage for you, " a fitting place for the mistress he wished to keep captive.

But he abandoned Theresa in France, where she became sick.

When she finally recovered, she returned to Scotland and met Yelverton at the Ship Inn, in Leith, where he urged her to leave Edinburgh and go to Glasgow, suggesting she might captivate another, wealthier man and live a happier, more stable life.

On 26 June, 1858, his brother visited her, claiming that Yelverton was waiting for her in Glasgow. It was a blatant lie. Earlier that day, in Edinburgh, Yelverton had married Emily Marianne Ashworth Forbes, in the Episcopal Trinity Chapel. She was the widow of a professor of natural history at Edinburgh University and Yelverton had been courting her for months. He remained "married" to her until he died in France in 1883.

A woman scorned, Theresa appealed to the public prosecutor of Edinburgh, accusing Yelverton of bigamy on the basis of the Irish marriage. He was suspended from the military. In 1860, the Scottish court combined the cases and delayed them. Meanwhile, Theresa appealed to the newly created Court for Divorce and Matrimonial Causes in England for restitution of conjugal rights.

For almost two years these cases started and stalled. In March, 1861, the Four Courts in Dublin found in her favour, but the following year the Scottish Court of Session in Edinburgh decided against her, although she later won on appeal. In England, the case went as far as the House of Lords, where she lost and her second appeal was dismissed. By 1870, Yelverton – who when accused in a Dublin court of being "very cool", coolly replied, "Not in bed" – had become the fourth Viscount of Avonmore; Theresa assumed the title of Viscountess Avonmore.

Meanwhile, she was becoming a self-made woman, restlessly searching for independence and making a living as a writer and critic. She travelled the world, moving back to Edinburgh to write a book about the US because she regarded it as her de facto home. She brought with her a young Sinhalese boy, an aide who had followed her up a mountain in Borneo. "Curious eyes watched them around Edinburgh," says Schama.

In the US, in 1869, she lived in the Yosemite Valley, where she was involved in a relationship that was perhaps her last real hope for companionship.

The man was John Muir, the Scottish-born founding father of American environmentalism, then a fortuneless, nomadic, unattached and unknown rambler. He knew her story and staunchly defended her innocence. She had been wronged, he assured her.

The hirsute Muir embodied Theresa's masculine ideal: the chivalrous savage she thought she'd found in Yelverton. But then Muir went off on his treks and failed to write to her. Although they communicated sporadically over the years, their connection never took on that all-consuming dimensions that her relationship with Yelverton had assumed almost from the beginning.

"Theresa progressed from broadsheet darling to tabloid victim, but she eventually became perhaps the first woman to turn unwanted celebrity into a journalistic advantage, playing off her fame to develop an audience, then pursuing her stories with no-holds-barred determination," writes Schama.

While writing her book, 25-year-old Harvard graduate Schama pored over archives, particularly in the British Library, as well as the bound volume of Theresa's letters. Schama also spent a month in Edinburgh, in the summer of 2004, while studying creative writing at the university. She'd wanted to take a break from her researches, "but, with the castle looming over me, the past seemed inescapable".

Steeped in the capital's unique atmosphere, she set out to find Theresa's St Vincent Street rooms and, irony of ironies, discovered that the townhouse was now the office of a law firm. "Theresa's life had been so entwined in legal proceedings that this seemed more than a coincidence," she says.

Some reviewers of Wild Romance have treated Theresa – who died in South Africa at the age of 48 in 1881 with 10 to her name – as cruelly as Victorian society once dismissed her, calling her "stupid, desperate and embarrassingly thick-skinned" or, worse, "a stalker", as if she were the Victorian version of Glenn Close's Fatal Attraction "bunny-boiler", Alex Forrest. Indeed, Schama acknowledges that had Theresa been a fictional character she would have been the madwoman in the attic, or a deluded character in one of the "sensation" novels of the era. A friend e-mailed Schama the other day to say that she still had 50 pages to read of the book, but that she loved the story. "Then she wrote, 'That Theresa, she was one crazy b*tch!'. And I guess that really does sum her up.

"In Theresa's words, 'Life, indeed, is a wild romance, if truly written.'"

Wild Romance: The True Story of a Victorian Scandal by Chlo Schama (Bloomsbury, 16.99).