Tourism officials on the Isles of Scilly are to lodge a complaint over a claim made by Jersey's tourism office in a £1m TV advertising campaign that the largest of the Channel Islands is "the warmest place in the British Isles".
Met Office temperature records for Scilly obtained by the Guardian also appear to undermine Jersey's claim.
The Met Office officially recognises Scilly as the warmest place in the UK. A Met Office spokesman said that Jersey – 50 miles further south – does not fall under its auspices because "it is not part of the UK", although "it had no reason to doubt" Jersey's claims to be warmer.
The small print on Jersey's advert says it bases its claim on "minimum temperatures supplied by the Jersey Meteorological Department". Tony Pallot, Jersey's principal meteorological officer, said Jersey's "mean minimum" for 1971-2000 – the period used by the Met Office to calculate all its mean temperatures – was 8.9C (48.2F).
However, Met Office data for Scilly seen by the Guardian says the "mean minimum" for St Mary's, Scilly's largest island, was 9.4C over the period. On other measures such as hours of sunshine and maximum mean temperatures, Jersey performs marginally better.
The council of the Isles of Scilly is also contesting the use of the term "Britain" in Jersey's advert, arguing that the Bailiwick of Jersey is a British crown dependency but not part of the UK. It also argues that, geographically, Jersey is not part of the British Isles archipelago.
Julian Pearce, the council's economic development officer, said: "We shall be writing to Jersey to remind them of both our geographical position and our ranking as the warmest place in the UK."
He also said he was investigating whether the council had grounds to make a formal complaint to the Advertising Standards Authority (ASA).
Jennifer Ellenger of Jersey Tourism said: "We stand 100% behind our advert and we have the data to prove we are the warmest place. We are politically part of the British Isles, even if not strictly geographically." She confirmed the advert's claim is based on the mean minimum temperature, but also on two further measures – average hours of sunshine per year and "mean annual" temperature. She added that another aim is to suggest the people of Jersey are the warmest in the British Isles, too.
Ordnance Survey, the national mapping agency for "Great Britain" – namely, England, Scotland and Wales – said it defines the "British Isles" as being "all the main and offshore islands of Great Britain and Ireland, as well as the Isle of Man and the Channel Islands".
Malcolm Bell, head of tourism at VisitCornwall, which works closely with Council of the Isles of Scilly to attract 4.5 million tourists to the far south-west of England each year – 100,000 ofwhom visit Scilly – said he would be writing to Jersey Tourism in support of Scilly. He said: "When I first saw the advert I just laughed. It seems a bit desperate to base an advert on warmth alone when people who are only interested in that would just go to somewhere such as Dubai instead.
"But the advert's claim is also a bit cheeky. People want honesty in advertising.
"Jersey might just about be technically correct in what they say, but it is bordering on unethical to stretch the truth like this when promoting your destination. We are in economically challenging times and we want to fight fairly for every customer."
Jersey, which received 685,000 visitors in 2010, has been challenged before over its tourism campaigns.
In 1990, the ASA upheld a complaint against Jersey after it failed to substantiate a claim that it had more sunshine that anywhere else in the British Isles.
In 1993, the ASA criticised Jersey for making exaggerated claims after it ran a newspaper campaign boasting that "our sandy beaches are always spotless and all of our golden beaches are scrubbed and rinsed twice by clear blue water".
The ASA noted that "on seven recent occasions large quantities of untreated sewage had poured onto bathing beaches".
Last year, Jersey Tourism accused the Met Office and the BBC of discrimination because it felt the Channel Islands were routinely left out of national weather forecasts. As a result, it said it was missing out on millions of pounds worth of revenue from prospective visitors.
"Many people don't have any idea just how lovely the weather is a short hop away from the mainland – meaning we lose out on valuable income," David de Carteret, director of Jersey Tourism, said then.
It was this grievance, says Jersey Tourism, that led it to base this year's campaign around the "warmest place in the British Isles" slogan.
This article was amended on 11 April 2011. The original said 8.9C is 32F. This has been corrected.
Scilly v Jersey
Number of islands
Scilly: 145 (five inhabited)
Jersey: 1
Population
Scilly: 2,100
Jersey: 91,626
VAT
Scilly: 20%
Jersey: 0%
Economy
Scilly: tourism, daffodil farming
Jersey: banking, tourism, Jersey Royal potatoes, Jersey cattle
History
Scilly: site of hundreds of shipwrecks, including five German U-boats during second world war
Jersey: occupied by the Nazis from 1940-45
Sport
Scilly: smallest football league in the world with just two teams, Woolpack Wanderers and Garrison Gunners
Jersey: fields its own team at the Commonwealth Games
TV highlights
Scilly: An Island Parish
Jersey: Bergerac
Famous visitors
Scilly: former prime minister Harold Wilson regularly holidayed on Scilly and is buried at St Mary's Old Church
Jersey: Alan Whicker lives on the island
Languages
Scilly: English, Cornish
Jersey: English, French and Jèrriais
Local food and drink
Scilly: pasty; "scuppered" ale
Jersey: "bean crock" with cabbage bread; cider