Impromptu verse written in London pub is 'in the vein' of Under Milk Wood and likely to fetch five-figure sum

An impromptu drinking ditty, dashed off in pencil by Dylan Thomas while seated at a London bar, is to be published for the first time after coming to light during the centenary of the poet's birth.

The ode to the pub was discovered by Fred Jarvis, a former general secretary of the National Union of Teachers, in papers belonging to his late wife, Anne, whose parents knew Thomas.

Written in Henneky's Long Bar – now the Cittie of Yorke – in High Holborn, London, it was described by one Thomas expert, Professor John Goodby of Swansea university, as "no masterpiece" but a very rare, exciting and "pretty valuable" find.

The "song", about the threat of a fabulous new hostelry run by "Mr Watts-Ewers / (Licensed to sell / Beer, wine and spirits / And tobacco as well)" was scribbled on the reverse of headed notepaper for the Apollo Society, a poetry and concert society whose original members included Peggy Ashcroft, Stephen Spender and Cecil Day-Lewis, and for which Thomas gave readings.

Beneath the letterhead is handwritten: "This little song was written in Henneky's Long Bar High Holborn by Dylan Thomas in 1951." It also bears the signature "Dylan Thomas, Esq".

In it Thomas, who had more than a passing fondness for pubs and drink, plays on the familiar round-buyers' refrain, "What's yours?" to describe the detrimental impact of "Mr Watts-Ewers' / Buckingham Palace of Booze" on rival publicans.

Jarvis, 89, found it among "piles of papers" while researching his autobiography, You Never Know Your Luck: some reflections of a Cockney campaigner for education, due to be published through Grosvenor House next week. He has been given permission by the poet's estate to include the previously unseen poem.

The manuscript was left to his late wife by her mother, Maureen. Jarvis said: "I think it is delightful. It's very exciting, especially in the centenary year. I am not a poetry lover, but I do love Dylan's work."

His wife's family knew the poet through their Oxford printing business and her father used to drink with him.

Goodby is now to include the song in a new edition of collected Thomas poems, to be published by Orion in October. "The handwriting is Dylan Thomas's. I have just spent several weeks in America looking at his manuscripts, so I am pretty au fait with his handwriting," he said.

Describing it as "a little pub song", Goodby added that it was typical of the "songs, lyrics and very simple straightforward ballads" Thomas was interested in before his death, aged just 39, and was "in the vein" of Under Milk Wood.

"It's very rare. Because it is Dylan Thomas, people didn't throw things away. It's over 60 years since his death, so most of what is out there has come on to the market already. There isn't much left, maybe one or two things."

It was "not intended" to be a masterpiece, and "nothing much by way of a poem", just a drinking song. "A bit of light amusement. He probably made it up on the spot", he said.

Of its potential value, Goodby said a two-sentence letter to the portreeve of Laugharne, the poet's Welsh home, had recently been auctioned. "It's just a boring letter saying, 'I am sorry I couldn't turn up to your festival,' from around 1952. That's all it says and I think it went for several thousand. So you can imagine what something like this would go for. I wouldn't be the person to consult, but you would be looking at five figures, I would have thought."