An appeal to raise £1.6 million to buy a portrait of the English poet John Donne has been launched by the National Portrait Gallery in London.
The 16th Century picture, regarded as the most important painting of Donne, has been privately owned for 400 years.
Current Poet Laureate Andrew Motion said the painting's "innate qualities" made it "indispensable".
It is thought to show Donne when he wrote one of his best love poems, To His Mistress Going To Bed, in 1595.
Andrew Motion and the Dean of St Paul's, Dr John Moses, helped launch the appeal to acquire the portrait.
Motion described Donne as "one of the greatest writers in the English language" and said the painting was "cheap at the price".
He added that the portrait was "broodingly suggestive of Donne's intellectual figure as well as his witty sensuality. It is also a picture of great intrinsic beauty and the bewitching evocation of an age".
The study of the Elizabethan poet, known for the lines "No man is an island" and "For whom the bell tolls", appeared on many Renaissance poetry book covers.
The artist behind the portrait is not known but it bears the inscription Oh Lady Lighten My Darkness, leading many to believe it was a love token.
After Donne's death in 1631 the picture was left to the first Earl of Ancram, whose family held on to it until the present day.
Sandy Nairne (L) and Andrew Motion discussed the painting
Lord Lothian, one of the earl's descendants, said the painting had "always been much loved by those of us who have lived with it".
He said he would be "delighted" if the gallery raised the money to buy it.
Gallery director Sandy Nairne said he was "very determined and confident" it would raise the funds.
The London gallery has six months to come up with a total of £1,652,000 for the portrait, which it is borrowing for its Searching For Shakespeare exhibition in March.
Charity The Art Fund has announced its contribution to the campaign with a grant of £200,000.