The daughter of upper-class Anglicans, Miss Palmer-Tomkinson studied with a rabbi for a year in preparation for her conversion to Judaism, a pre-requisite for her marriage to Mr Sebag-Montefiore, scion of two grand old families of Anglo-Jewry.
The rigorous instruction required for converts to the Jewish faith did not deter her. She told the Jewish Chronicle that she had found it an "interesting and inspiring" experience.
"As well as learning with the rabbi every other week for three hours, I had to write five essays of over 4,000 words and then I had to sit in front of a rabbinical board," she said.
Miss Tomkinson is not alone. Increasing numbers of people are converting to faiths whose clerics demand not just an interest in their beliefs, but time and commitment. Judaism, Catholicism, Islam and Buddhism have all seen the numbers of converts go up in recent years, often prompted by marriage into the religion but at other times reflecting a deep desire for a faith which gives life renewed meaning.
As society becomes more open, conversions are now an important religious trend, according to Rabbi Jonathan Romain, a leading authority and writer on the subject.
He said: "A major religious undercurrent is flowing through society in which thousands of people are converting from one faith to another. The established Church may be suffering a crisis in membership, but faith is far from being in decline."
Miss Palmer-Tomkinson, 27, shy sister of "It Girl" Tara, will marry Mr Sebag-Montefiore, a journalist, at the Liberal Jewish Synagogue (LJS), in St John's Wood, the house of worship favoured by many of the country's Jewish celebrities. The guest list includes both the Prince of Wales and Camilla Parker Bowles - if the gossip columns are to be believed - and the attendant publicity could do for Judaism what the marriage of Imran Khan and Jemima Goldsmith - daughter of the half-Jewish tycoon Sir James - did for Islam.
Judaism, which is a non-proselytising creed, has had several thousand converts this decade. One is the television film-maker Desmond Wilcox, who became a Jew six years ago. He and his wife, the television presenter Esther Rantzen, will enhance the LJS's reputation as the "showbiz" synagogue when they wed there in December - 25 years after their first marriage in a register office. They will make their vows standing under the chupah - the flower-bedecked canopy erected for Jewish weddings.
Catholicism has always had its share of high-profile converts - among them the Right Rev Hugh Montefiore, the Roman Catholic Bishop of Birmingham, who converted from Judaism and is a relative of Santa Palmer-Tomkinson's husband-to-be.
Other high-profile recent converts include the Duchess of Kent, Frances Shand-Kydd, mother of the late Diana, Princess of Wales, and Tory MP Ann Widdecombe. Recruitment reached a peak after the Church of England decided to ordain women, and entire congregations and their vicars converted.
More than 50,000 people have received instruction in the past few years, which involves attendance at classes where the faith and the gospels are studied. The number of conversions due to marriage may well increase after last week's statement from the Roman Catholic bishops that non-Catholics, such as those in mixed marriages, who attend mass cannot automatically receive communion.
Muslims are being converted at the rate of 1,500 a year. The conversion ceremony - the Shahada - requires the would-be convert to bathe, dress in clean clothes, gather Muslim witnesses and recite a prescribed testimony to God.
The Buddhist credo with its spiritual and meditative aspects has attracted individuals alienated from Western secular values. From hippie-linked beginnings it is now practised by more than 120,000 people in the UK.
The toughest road to conversion is the one taken by aspiring members of Orthodox Judaism. Its rabbis insist that would-be converts go and live with a pious Jewish family for months or even years, and immerse themselves in the minutiae of the close-knit community before they are accepted.
Orthodox Jewry does not even recognise conversions from the progressive Jewish congregations such as the LJS, viewing them as ersatz Jews. Indeed, the issue is the exposed nerve of contemporary Judaism.
A woman whose husband converted under the Orthodox London Beth Din (Jewish religious court) said: "The entire process is shrouded in mystery and can go on for years, and it involves an enormous amount of emotional stamina."
FAITH TO FAITH
Ann Widdecombe, Conservative MP - Anglican to Catholic
Mike Tyson, boxer - Christianity to Islam
Alan Clark, Conservative MP - Anglican to Catholic
Roberto Baggio, footballer - Catholic to Buddhism
Felicity Kendal, actress - Anglican to Judaism
Lynne Franks, PR agent - Judaism to Buddhism
Charles Moore, editor of the Daily Telegraph - Anglican to Catholic
Leonard Cohen, singer and poet - Judaism to Buddhism
Frances Shand Kydd, mother of Diana, Princess of Wales
- Anglican to Catholic
Nick Howard, son of former Home Secretary Michael Howard - Judaism to Christianity
John Gummer, Tory MP - Anglican to Catholic
Bob Dylan, musician - Judaism to Christianity to Judaism to Buddhism