As one Game of Thrones ends, another begins: eight years after Hilary Mantel left her protagonist Thomas Cromwell apparently triumphant after the beheading of Anne Boleyn, he is due to strut back into the literary arena in The Mirror and the Light.

Publisher HarperCollins announced on Wednesday morning that the final novel in Mantel’s trilogy of historical novels about the life of Thomas Cromwell, will be published in March 2020. The long-awaited novel will cover the final four years of Cromwell’s life, starting with Boleyn’s execution in 1536, and moving to his own execution for treason and heresy in 1540.

It follows Wolf Hall (2009), which charted Cromwell’s rise to greatness, and Bring Up the Bodies (2012), covering the machinations that led to Boleyn’s beheading. Both novels won the Booker prize and have sold a combined 1.5m copies, making the third one of the most anticipated novels of the decade.

The author said she had been on a “long journey” with her protagonist, and hoped her readers would follow her with him to the end of his life.

“When I began work on my Thomas Cromwell books back in 2005, I had high hopes, but it took time to feel out the full scope of the material. I didn’t know at first I would write a trilogy, but gradually I realised the richness and fascination of this extraordinary life,” she said. “I hope they will stay with me as we walk the last miles of Cromwell’s life, ascending to unprecedented riches and honour and abruptly descending to the scaffold at Tower Hill. This book has been the greatest challenge of my writing life, and the most rewarding; I hope and trust my readers will find it has been worth the wait.”

Her publisher Nicholas Pearson at Fourth Estate said The Mirror and the Light was “every bit as daring and thrilling” as the novels that precede it.

“Completely immersive, as it charts the final years of Cromwell, it also casts a fresh light on the politics of power and the way we live now,” said Pearson. “When I first read the opening pages of Wolf Hall, I was drawn into history in a way I hadn’t experienced before. Hilary Mantel allows us to inhabit the past as it happens – we watch Cromwell and those in his orbit as their lives unfold, making decisions the consequences of which they cannot know.”

HarperCollins said the novel would offer “a defining portrait of predator and prey, of a ferocious contest between present and past, between royal will and a common man’s vision: of a modern nation making itself through conflict, passion and courage”. It also confirmed that the novel would be adapted for television by the BBC, following the Bafta-winning adaptation of Wolf Hall starring Mark Rylance as Cromwell. Peter Straughan will write the adaptation, and Peter Kosminsky will direct. A film exploring the life and work of Mantel herself, from Oxford Films, is also due out next March.

In 2017, in response to questions about the years between the second and third instalments, Mantel said the expectations of her readers were slowing her down. “People ask me if I’m having trouble killing off Thomas Cromwell. No, why would I?” said the author. “It is 10 years’ worth of effort and it is lovely to have the encouragement of people who are waiting for it, but that’s why I want to deliver them something that is the very best.”

The announcement follows intense speculation about the book’s launch after the sudden appearance of a new billboard in Leicester Square on Tuesday. Emblazoned with the Tudor rose used on the cover of Wolf Hall, as well as the novel’s opening words – “So now get up” – the advertisement disappeared shortly afterwards.

Waterstones Piccadilly (@WaterstonesPicc) Erm... grabbing lunch and saw this in Leicester Sq! That rose. That line. Oh my god is the new Hilary Mantel finally coming?!?! pic.twitter.com/5tytsBqQg7

Waterstones fiction buyer Bea Carvalho said The Mirror and the Light was the book hopeful customers asked about most frequently. “It is set to be one of the biggest releases of 2020 and indeed the decade to come,” she said.