As people around the world prepare to sink thousands of gallons of the country's most famous export on St Patrick's Day this week, the startling claim that we should be tracing leeks rather than shamrocks on our pints has been made by the inhabitants of a Welsh town.

Locals from Llanfairfechan, near Bangor, claim to have identified the site of a tavern from where they claim Arthur Guinness stole the recipe for his famous stout in the 1750s. The Guinness company was this weekend unable to discount the theory.

In a twist straight from the firm's advertising campaign - 'Not everything in black and white makes sense' - the Welsh claim suggests that Guinness should be rechristened Gynwys. The drink is made from hops, barley and its secret ingredient - a special yeast. The Irish were once able to claim the drink derived its magical, life-enhancing properties from the addition of water from the River Liffey rather than its 4.1% alcohol content, but today Guinness is a global brand brewed in 50 countries.

The company's official history says Arthur Guinness developed his taste for dark ales on visits to London where he drank a brew called porter, named after the porters of Covent Garden and Billingsgate with whom it was especially popular. At the time, porter was almost certainly dark brown rather than black.

It has been suggested that before Guinness established his brewery in Dublin in 1759 he may have converted the Irish to porter by importing it from London. But Welsh local historian Deiniol ap Dafydd says it would not have made commercial sense for Arthur Guinness to transport porter all the way from London: the costs would have put the drink beyond most people's pockets.

And, he adds, it would have been a risky undertaking. In the eighteenth century the two-day crossing to Dublin was made from Holyhead on Anglesey. But before the construction of bridges at Conwy and the Menai Strait, Holyhead could only be reached by traversing the mountains of Snowdonia - a superhuman feat with a cargo of drink.

The claim has been given added weight by the identification of the site where Arthur Guinness is believed to have stopped on the last leg of his journeys to Holyhead. Two cottages are still standing - one known as Llety, the Welsh word for place of rest, was where the mail coach stopped overnight, and the other was then a tavern where a local black ale was brewed. That building is still known by its original name - Gwyn du, or 'black wine'.

Ap Dafydd said: 'The ale was renowned for being much darker, smoother and deeper than porter - much more like what we know today as Guinness.

'My view is that Mr Guinness tasted a whole range of local brews on his journey between London and Holyhead, but it was the black wine that took his fancy. It is what was to become Guinness.'

Peter Edmondson, who runs the garden centre in Llanfairfechan, adds that old maps of the town show three malting sites used by the pub. 'They would have produced far more malt than could ever have been needed in what was then a village of a few hundred people.' The surplus could have been going to Ireland.