In Chaucer's city, Sarah Shuckburgh is mesmerised by the superb cathedral, but also intrigued by a remarkable woman.

Canterbury is full of streets, shops and hotels named after Chaucer, Thomas Becket, St Augustine and Christopher Marlowe, and the city has many other famous sons, including Orlando Bloom, Bagpuss and Rupert Bear. But by the ruins of St Augustine's abbey stands a newly unveiled statue of an extraordinary woman whose contribution to the history of Canterbury and of England has been all but forgotten for 1400 years. The half-ton bronze figure is larger than life. The sculpted face is powerful, but calm and demure beneath its Frankish headdress. A second statue, nearby, represents her husband, a pagan warrior king who has come to meet her as she returns from church, with news that the missionary Augustine has landed on the Kent coast. Scholars now believe that Pope Gregory sent St Augustine to Canterbury because of this woman. I had scarcely heard of Queen Bertha before, but I was immediately intrigued by the idea that she was the reason that Canterbury became the centre of Christianity in England.

A short walk took me from the abbey ruins to St Martin's, the church from which Bertha was returning - the smallest and oldest of the three religious buildings which now form Canterbury's World Heritage Site. Bertha was the first Christian to worship here, in about 580AD, and, uniquely, the parish church has been in continuous use ever since. Bertha's Saxon husband Ethelbert - later King of Kent - was a pagan who worshipped Thor and Wodin - but Bertha was a Christian, daughter of Frankish King Charibert of Paris. On their marriage, Ethelbert promised that she could continue to practise her religion. He provided an old Roman mausoleum for her worship, and Bertha dedicated her new chapel to the 4th century St Martin of Tours, the city where she had grown up.

You can still see the Roman bricks and tiles of the ancient church, and the stone lintel above an original entrance to the chancel. Beside it, a small arched doorway, added in Saxon times but now bricked up, is probably the one which Bertha used each day.

By 597, the queen had worshipped here for 17 years and Ethelbert must have become familiar with her beliefs - thus, unlike other Saxon kings, he was prepared to welcome a missionary from Rome. Augustine and his 40 companions based their mission at Bertha's small chapel, and added the brick and stone nave and buttresses, and also the windows in the west wall which are today only visible from inside the church.

Ethelbert probably took two or three years to make the momentous decision to abandon his traditional gods, and to accept the religion of a foreign power in his kingdom. But, encouraged by Bertha, he eventually became a Christian - his baptism starting the conversion of the English, and setting the pattern for the whole of medieval Europe. I wondered whether Ethelbert was baptised in the ancient stone font in St Martin's church, but disappointingly it is an old well-head from the 12th century - 500 years after Bertha's era.

Reluctant to return to 21st century fast food, I had lunch in the Goods Shed at Canterbury's West Station - Britain's only daily farmers' market. Beneath a high beamed roof, stalls offer locally grown flowers and vegetables, organic meat, freshly caught fish and crabs and home-baked cakes. I bought some traditional Kentish cider, and tasted six blends of apple juice before deciding on a cloudy green Cox-Bramley. I walked off the wild salmon steak and lemon posset pudding by striding along the city walls, which follow the fortifications of the egg-shaped Roman city of 300AD - walls which Queen Bertha would have known. Canterbury's shopping streets teemed with people, but away from the neon signs, the cobbled lanes of medieval houses were almost deserted.

I was the only visitor in the Museum of Canterbury. The building was once the poor priests' hospital, a medieval retirement home for clergy. Below lie the stone foundations of a minter's house built in 1174 - when Canterbury had its own currency. Thomas Becket had been murdered in the cathedral just four years earlier, in 1170. But this chapter of Canterbury's history was too recent for me - my quest was for Queen Bertha, who nurtured the earliest roots of English Christianity. In the Anglo-Saxon gallery, I spot an exquisitely crafted gold pendant, decorated with garnets and a Christian cross, made for a wealthy or noble woman in Bertha's day. Artists' impressions depicted Canterbury as it might have looked before Bertha arrived - thatched huts and orchards among Roman ruins, a town populated by illiterate heathens - and of the city after Ethelbert's conversion, with Saxon cathedral and monastery.

That evening as I walked up the High Street, starlings jabbered in the lime trees, and music blared from crowded pubs and cafes. Diners filled restaurants serving food from Thailand, Morocco, China, Italy and Spain. Hen parties of girls wearing wings and tiaras staggered through the old Buttermarket, and the cobbles rang with the click of high heels and good-natured heathen shouts.

On Sunday morning, cathedral bells summoned worshippers to Matins and gulls squawked from the grey sky. I strolled through a maze of narrow lanes, past converted oast-houses and cottages with tiny crooked doors, overhanging upper storeys, timbered walls and undulating roofs. I lingered at Queningate - the Queen's gate - an archway leading to the cathedral precincts. Nearby are the remains of the original Queningate, a Roman postern gate through which Queen Bertha walked on her way to St Martin's.

Finally, I entered the cathedral. A brass compass-rose, set into the floor of the nave, marks the probable site of St Augustine's original cathedral, founded by Ethelbert. Christ Church was rebuilt after the Norman Conquest, and extended over the next three centuries. The result is breathtaking. After gazing at the immense sunlit nave, raised quire, side chapels, pulpit screen, crypt and ancient tombs, I wandered round the outside of the cathedral, over velvety lawns, through shady cloisters and beneath gleaming masonry.

Back in the Buttermarket, I sheltered from a sudden shower in an overhanging medieval building (now a café). As I sipped my cappuccino, I heard an amplified Latin chant, and a fainter sung response, and across the cobbles towards Christ Church Gate came a procession of pilgrims who had walked from Ethelbert's cathedral in Rochester to worship in the ruins of St Augustine's abbey.

At three o'clock, the cathedral bells pealed again, and drew me into Evensong. For forty-five minutes, agnostic as I am, I was soothed and exhilarated by the music flooding the high vaulted ceiling, by the stained glass, gemlike in the afternoon sunshine, and by the curious and unlikely beliefs, which since Bertha's day, have underpinned English laws and customs.

The quire was full of Chaucerian characters. Sitting next to me was a gap-toothed Wife of Bath, plump and jolly, reciting the creed loudly and a word later than everybody else. Across the aisle, an aquiline-nosed Knight whispered to a curly-headed Squire. In another pew, a burly tee-shirted Miller sported a beard and red cheeks. A mobile phone bleeped during the anthem, and I glanced at its owner - a thin-faced Pardonner with pale, wispy hair. But here also were characters unfamiliar to Chaucer - a demurely dressed Muslim couple, a group of Chinese tourists, bulky Americans in shorts, Japanese youngsters with dyed hair, a cross-section of modern multicultural society. And after the final organ voluntary, this diverse congregation burst into applause. St Martin's church, the cathedral and the abbey ruins - where Bertha, Ethelbert and Augustine were buried - are only part of the enduring legacy of the queen who brought Christianity to England. But Canterbury - headquarters of the church, prosperous city and tourist centre - owes everything to Queen Bertha.

First published by the Telegraph
©SarahShuckburgh