Sensational details of the tangled love affairs, gambling debts and secret children at the heart of the British establishment are to lift the bedcovers on late-18th century aristocracy, according to the British Library, which is buying the personal papers of the influential Granville family.

The previously censored private correspondence was stored away from the Granvilles’ main political archive in sealed tin boxes at Devonshire House in London. Among the revelations is a candid letter sent in 1809 to Harriet Granville, daughter of the celebrated socialite Georgiana, Duchess of Devonshire. Written by her aunt, who was also called Harriet, it informs the younger woman that her dashing new husband is the aunt’s former lover and father of her two illegitmate children.

“I lov’d this Man to distraction; gave him every proof of love, Woman could give – ; dreading the misery my disgrace would bring on my family, or My poor Mother; My Children – I conceal’d my guilt… Ah Harriet, I have given birth to two wretched little beings, who will bear thro’ life, the disgrace My Crime has brought upon them. – Now all is told!” she writes in a letter shown to the Observer.

The Duchess of Devonshire and Lady Bessborough (then Duncannon) in an 1782 engraving by William Dickinson,. Photograph: Getty Images

Harriet senior is author of 1,000 of the freshly disclosed letters. Born Lady Henrietta Frances Spencer in 1761, she is an ancestor of the late Diana, Princess of Wales, and had a life as glamorous and dramatic as her better-known, wealthier sister, Georgiana, played by Keira Knightley in the 2008 film The Duchess.

At 19, Harriet had married Viscount Duncannon, later third Earl of Bessborough, but she conducted a serious 17-year love affair with Lord Granville Leveson-Gower, a younger dandy blessed with brilliant blue eyes into which a middle-aged Lady Bessborough later declared she had “looked her life away”.

Writing to him at the height of their love, she said: “‘I never deceiv’d myself from the first moment I let you press me to your bosom without resistance, I recokn’d myself as much yours, as if I had liv’d your mistress for years.”

But her letters also chronicle the emotional pain she went through at the births of their secret children: a daughter and a son. In August 1800, she wrote to Granville: “As soon as Mrs. Norris return’d she took the child &; was secreted in Sally’s room &; lock’d in while I fainting &; dying was dragg’d back [as I came] to my own bed which Sally had prepared.”

Bessborough kept both births hidden, although living with her husband. The children, Harriette and George, were taken in by her sister Georgiana’s household.

These dramatic letters are typical of the stash, according to library curator Laura Walker, and they offer new insights on a close-knit group of women, each married to a government minister, prominent land owner or ambassador.

“This wealth of correspondence sheds light on that whole class,” said Walker. “From the marital aspects of their lives to the gambling debts they got so much into, it really paints a picture that was missing. They were in a world of privilege but they couldn’t escape their marriages or see their illegitimate children.”

Keira Knightley as Georgiana, Duchess of Devonshire, in the 2008 film. Photograph: Allstar/BBC

Bessborough was also briefly the lover of the renowned playwright Richard Brinsley Sheridan, author of The Rivals, and she was the mother of the writer Lady Caroline Lamb, now most famous as Lord Byron’s lover and wife of the Victorian politician who became Viscount Melbourne.

After Georgiana’s death in 1806, the Duke of Devonshire married his long-term mistress and the cache of letters, shown to the Observer in advance of their acquisition by the library with the help of a £865,200 grant from the National Heritage Memorial Fund, shows the dutiful aunt then sought a suitable husband for her unhappy niece. Top candidate was her own great love, Granville, then 37. Still besotted, Bessborough calculated the marriage would keep him in her circle.

“The political papers have been studied before but what has been seen of the personal correspondence was heavily edited in the Edwardian period,” said William Frame, head of modern archives and manuscripts at the British Library. “You can see from their emotional language these women belong much more to the Romantic era. It was very different to the Victorian era that followed.”

Additional research by Jason Rodriguez