Princess Beatrice rings few bells. But in fact she was the last of those nine dough-faced, light-eyed children born to Queen Victoria and her Albert during their surprisingly frisky marriage. Victoria hated babies but didn't seem able to stop making them. "You are very wrong in thinking that I am not fond of children," the Queen once wrote crossly to her eldest daughter. "I admire pretty ones immensely," which seems slightly to miss the point. Only the use of chloroform during her last couple of confinements reconciled her to being what she insisted on coding in French - so horrible was the idea in plain English - "mamma d'une nombreuse famille".

Beatrice has had biographies all to herself before, most memorably David Duff's 1958 classic The Shy Princess (there is not a lot of wiggle room when it comes to thinking up titles for someone no one has heard of). Duff's Beatrice was a gentle soul, a kind of Angel in the House, or Houses, who tiptoed around Osborne, Sandringham and Balmoral plumping cushions and making sure that Mama had everything she needed. Dennison's Beatrice, by contrast, is a steelier creature, a gate-keeper who controls access to the most powerful person in the world and tells prime ministers when it is time to leave.

It didn't start like that, though. Born when her parents were already middle aged, Beatrice was destined to be the precocious pet of the royal nursery. Always known as "Baby", she was allowed a latitude that would have been unthinkable for her elder siblings. While they had been drilled in Latin and military exercises, Beatrice wore pretty frocks and lisped nursery rhymes at cousins visiting from marshy corners of northern Europe.

All the petting stopped suddenly, however, at a quarter to 11 on the night of December 14 1861. With the death of the Prince Consort, Victoria famously crossed over into a land of grief and mourning from where it proved impossible to grab her back. As her only companion she took the four-year-old cherub, whose main occupation until then had been charming the grown-ups. At a stroke four-year-old Beatrice was transformed from an adorable doll into a slightly musty comfort blanket.

Spending the next 25 years being chewed to bits by Victoria cannot have been fun. "Youngest daughters have a duty to widowed mothers," intoned Victoria, and set about sucking out the spirit that had marked Beatrice's early years. The princess's slender frame became buried under a bodysuit of fat, the auburn hair lost its glint and rheumatism swelled and stiffened the royal joints into premature middle age. Victoria did everything to make sure that Beatrice would never leave her. A fledgling romance with Louis Battenberg was quickly dispatched by sending the young man abroad; on another occasion, the Queen suggested that Beatrice might like to marry her late sister's widower as a way of preserving the status quo (the fact that this would have been illegal was only a minor inconvenience).

And yet, within this tightly prescribed sphere, Beatrice did manage to carve out a kind of life for herself. It was she who sorted the Queen's correspondence from cabinet ministers each morning, and sent out the replies. As the old lady grew frail and stiff, it was Beatrice who deputised for her at interminable ceremonies to plant trees and open bazaars. The popular press, finding it difficult to like the wayward Prince of Wales or the now-Prussian Crown Princess, settled instead for the homely Beatrice, who managed to embody what the previous five decades of Victoria's suety reign had been all about.

In the end Beatrice did manage to break free by marrying a surprisingly dashing prince from Spain, who agreed to set up home at the British court. The joy soon went out of the marriage when it became clear that their second son was suffering from haemophilia, the consequence of Beatrice's cursed genetic inheritance. Just 10 years later, and enervated by his role as a Ruritanian page to an ageing mother-in-law and her doggedly devoted daughter, Prince Henry set sail for an undignified little war on Africa's Gold Coast, where he succumbed to malaria.

Victoria's death in 1901 did not mean that Beatrice was finally free. She had been charged by the Queen with the task of going through decades' worth of journals and carefully carving out an acceptably anodyne version to put before the British people. Beatrice slashed and burned as she went, consigning her mother's original stinging words to oblivion and neatly stitching together something altogether sweeter. Matthew Dennison works hard to suggest that Beatrice's vandalism may not have been as wholesale as earlier scholars have suggested, but still it is hard not to see her as the woman who, gently but firmly, tore the tongue out of Queen Victoria.

This is an old-fashioned biography about an old-fashioned subject. At a time when non-fiction writers are desperately thinking up fancy new ways to tell stories, there's something rather comforting about a narrative that has no embarrassment in starting at the beginning and trundling through to the end. Dennison is careful to correct some of Duff's gossipier anecdotes and adds a slightly stiffer backbone to the story of the princess who has tended to go down in history as a tasty feast for her vampiric mother. If Dennison's Beatrice doesn't exactly step out into the spotlight, at least she is now clearly discernible as a power behind the throne.

· Kathryn Hughes's biography of Isabella Beeton is published by Harper Perennial