Only 22 of the belts were ever made but the theft has come at the same time as a string of recent sales in which several have come up at auction, including three given to Henry Cooper.
Petersen (1911-90), who won his belt outright in 1935 for three victories, had turned professional in 1931 and his successes included the British light-heavyweight and heavyweight titles. He defended the belt eight times but had to retire at the age of just 25 because of an eye injury.
His Mappin & Webb belt was taken from his son John's home in Burnham, Buckinghamshire, sometime between January and May 13, where it had been kept with other memorabilia from his illustrious career.
John told the South Wales Echo that they were waiting for the day when they could donate the belt to a sports museum in Cardiff, where his father was born and many of the family still live.
"I sincerely hope that they have not taken it to somebody who would just melt the thing down for the gold; that would be a disaster," John told the newspaper.
Investigating officer PC Ian Williams said it is a National Sporting Heavyweight Championship Belt (Lonsdale Belt), 9CT gold with enamel plaques. It has previous winners of the British title engraved on it, including "Bombardier Billy Wells" and "Jack Petersen".
"It was valued in 2007 by insurers at £20,000, although it is believed to be worth more than this due to the link with Petersen," PC Williams added.
The belt was contained within a dark blue box with a white/cream-coloured silk lining. The box is the length of the belt and approx 3in (7.5cm) deep.
Henry Cooper's Belts
Two of Cooper's Lonsdale Belts were sold by Bonhams Chester on February 20 for £10,000 each, while his first belt had been sold by them a year ago for £40,000. In 2011 Fellow's of Birmingham sold the 1926 belt awarded to 'Gentleman' Jack Hood for £36,000.
And later this month, on June 19, Bonhams Chester are offering the belt won by Sergeant Johnny Basham for his fights in 1914-16 with an estimate of £30,000-35,000.
Anyone with information about the theft of the Petersen belt should contact Thames Valley Police on the national 101 phone number and either quote the crime reference number, GD/4254749/13, or ask to speak PC 56 Ian Williams at Taplow Local CID. You can also contact Crimestoppers anonymously on 0800 555 111.
