A meandering street in north-west Wales that challenges the fittest of walkers and cyclists has been confirmed as the steepest in the world.

Ffordd Pen Llech in the historic town of Harlech – better known for its castle and rousing song, Men of Harlech – has been judged steeper than Baldwin Street in Dunedin, New Zealand.

Residents have long campaigned for the Guinness World Records title and are planning a huge party to celebrate the accolade.

It has been established that the street has a gradient of 37.5% at its steepest point, compared with Baldwin’s Street mere 35%.

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Gwyn Headley, who led the town’s campaign to claim the title, said: “I feel utter relief and jubilation. I feel sorry for Baldwin Street and the New Zealanders, but steeper is steeper.”

Headley expressed regret at the timing of the announcement, following hard on the heels of New Zealand’s unfortunate loss in the Cricket World Cup final to England (and Wales – the governing body is the England and Wales cricket board) but said: “At least they have the Rugby World Cup ... for the moment.”

Winning the title was a lot tougher than the townsfolk had anticipated. “Guinness World Records was ultra-specific in the criteria demanded for it to qualify as the steepest street in the world, and although we were confident in meeting or exceeding nine of them, we were worried about the 10th,” said Headley.

The 10th criterion was that Guinness World Records required a blueprint of the street. The Harlech bid justified its absence because the street has been there since time immemorial, or at least 1,000 years, before there were such things as blueprints.

A surveyor called Myrddyn Phillips, an expert on mountain measuring, did much of the hard work. He used a combination of hi-tech (a satellite dish) and low-tech (chalk to mark out key points and bricks to keep a tripod steady) to take a series of measurements on the street. At one point a volunteer dropped a brick and was astonished to see it rolling down the hill.

To qualify for the title, the street or road also must be a thoroughfare that is commonly used by the public, who are able to drive vehicles across it. Ffordd Pen Llech is flanked by 300-year-old houses and an ancient route to the castle. Motorists do use it – and the unskilled often become unstuck.

Located in an otherwise quiet valley of the South Island city, Baldwin Street in Dunedin has attracted daredevils and adventure sports enthusiasts, prompting the local council to upgrade infrastructure and residents to launch cottage industries selling food, drinks and souvenirs.

Some in New Zealand have appeared rattled, with one person suggesting it may be worth resurfacing Baldwin Street for the sake of the title, which has turned the neighbourhood into a tourist attraction.

Craig Glenday, the Guinness World Records editor-in-chief, said: “The local community in Harlech has shown sheer willpower in their quest to earn Ffordd Pen Llech the title. We know the anticipation has been building for quite some time now and I’m pleased to see the outcome has brought such joy to the residents.