A RARE poem by Robert Burns – that gave the tune to the Christmas carol Away in a Manger – is expected to fetch £20,000 when it goes under the hammer at auction later this year.

The hand-written manuscript of Afton Brae was penned by Burns in 1789.

Now known as Sweet Afton, the poem is considered one of Burns's most beautiful lyrics, and is one of the best-loved.

It romantically describes the Afton Water, near the bard's home in Cumnock, Ayrshire, and his sweetheart Mary sleeping by its "murmuring stream".

The poem was set to music by Jonathan E Spilman in 1837, under the title Flow Gently, Sweet Afton – now familiar to all carol singers as the tune to Away in a Manger, which was first published in 1885.

The rare manuscript will be sold by Bonhams in London in May, as part of the Roy Davids Collection – "the finest collection of poetry ever to come to auction". The manuscript includes the bard's own corrections, such as the species of bird – the stock-dove "whose echo resounds through the glen" and the blackbirds whose "chant in yon wild thorny den".

Meanwhile, a rare signed copy of a poem by the late Poet Laureate Ted Hughes is being auctioned to raise funds to fund research into the plight of wild Scottish salmon. A limited edition of Hughes's tribute to the salmon – The Best Worker in Europe – is being auctioned for the Perth-based Atlantic Salmon Trust with a guide price of £200.