"Last Letter" is a previously unknown poem by Ted Hughes, about the night his estranged wife Sylvia Plath killed herself.
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The poem, "Last Letter" was published in full in the New Statesman.
It begins: "What happened that night? Your final night" and goes on to chronicle the last few days before the former poet laureate's wife Sylvia Plath killed herself in February 1963, aged 30.
The New Statesman discovered the poem in the British Library archive, with help from Mr Hughes' widow, Carol Hughes.
Ted Hughes died in 1999. His last collection - "Birthday Letters" - was published in 1998 and broke his silence, held since Miss Plath's death, over their life together and his life after her suicide.
"Last Letter" was the poem he did not include in the collection.
Part of the poem imagines, or attempts to, the emptiness of Miss Plath's final hours.
"What happened that night, inside your hours
Is as unknown as if it never happened.
What accumulation of your whole life,
Like effort unconscious, like birth
Pushing through the membrane of each slow second
Into the next, happened
Only as if it could not happen
As if it was not happening."
Miss Plath killed herself a few months after Mr Hughes left her and their two children - Frieda and Nicholas - for another woman. Before gassing herself, Miss Plath made sure the children were safe and left them food and drink.
Six years later, the woman Mr Hughes left with - Assia Wevill - also killed herself and their four-year-old daughter, Shura.
'The darkest poem he has ever written'
Poet Laureate Carol Ann Duffy told Channel 4 News Last Letter was "almost unbearable to read."
"It feels a bit like looking into the sun as it's dying," she said.
"It's a poem of deep complicated feelings and in some ways it’s the heart of Birthday Letters. I think its absence from that original collection makes the collection more powerful. It stands, for me, as a poem on its own.
"It's a poem that will speak in the way that a Shakespearean tragedy does to people who've had the misfortune to touch on those issues. It shows how a suicide can scar the lives of those who still have to live after that death.
"It seems to me to be the darkest poem that he wrote about the death of Sylvia Plath. There is a kind of deafening agony, blinding agony to this new poem. It seems to touch a deeper, darker place than any poem he's ever written."
In 1984, Mr Hughes was appointed the UK's poet laureate and is considered to be one of the twentieth century's greatest English poets.
The newly released "Last Letter" shines more light on the tragic personal life that lies behind his poetry. Its final lines recall the moment he discovered Ms Plath had died.
"And I had started to write when the telephone
Jerked awake, in a jabbering alarm,
Remembering everything. It recovered in my hand.
Then a voice like a selected weapon
Or a measured injection,
Coolly delivered its four words
Deep into my ear: 'Your wife is dead.'"
Channel 4 News's Jon Snow on poetry
"Sylvia, Ted, Byron, Shelley, Keats...Where does the list of pain, agony, ecstasy, and the rest end?"
Read more on Jon Snow's blog on poetic justice.