Why Anne Boleyn lost her head for Henry VIII

A love letter written by Henry VIII to Anne Boleyn reveals how he pursued her like a lovesick schoolboy, declaring his "unchangeable intention" to marry her and signing off with "H seeks A B no other Rex" – his beloved's initials in a heart.

The romantic epistle, which eventually led Anne to accept his hand in marriage – something of an error on her part – has been locked away in the Vatican since being stolen more than five centuries ago. It was used by the Roman Catholic Church as evidence against granting Henry a divorce from his first wife, Catherine of Aragon.

In April, it will be exhibited in Britain for the first time as part of the British Library's show, Henry VIII: Man and Monarch.

All too aware of the monarch's reputation as a notorious womaniser, Anne continually frustrated him by refusing to become just another royal mistress. Instead, she held out for marriage. This letter marked a turning point in their relationship, with Anne at last appearing to make a "humble submission" to Henry's love.

In the entreaty, he assured her that "henceforth my heart will be dedicated to you alone" followed by his wish "that my body was so too". Utterly besotted, Henry apologised profusely for having suggested Anne could ever have been a mere mistress, and pledged to "pray once a day" for the circumstances which would lead them to consummate their love.

It all ended badly for Anne, who was beheaded at the Tower of London in May 1536.

The Vatican love letter joins other loans, among them portraits, tapestries and armour, that will be exhibited alongside the British Library's collection of correspondence, official documents, maps and books, to examine the extraordinary social transformations during the reign of Henry VIII.

Advance tickets for the exhibition go on sale from Monday.