A priceless sheet of the world's first adhesive stamp, the Penny Black, is set to leave the UK for the first time on 25 May. The sheet is one of a mere handful left in the world, all of which are kept at The Postal Museum in London.

Stored in a secure, humidity controlled, alarmed vault, the 176-year-old stamps are so rare that they are transported under armed escort in a specially designed bomb-proof case. The case is fitted with shock alarms and sensors so that the Museum can check on the condition of the stamps throughout the journey. The stamps are being transported to New York, for the World Stamp Show, which happens once every decade and is expected to bring in a quarter of a million visitors. After the show, the Penny Blacks will go on permanent display in London, where The Postal Museum will open its doors in spring 2017.

The Penny Black
One of the only surviving sheets of the world's first stamps, the Penny Black, is inspected prior to leaving the UK for the first time at The Postal MuseumMiles Willis/ Getty Images for The Postal Museum

The Penny Black was the first of its kind to be used in a public postal system. It was first issued in the UK on 1 May 1840, for official use from 6 May of the same year. The concept of using an adhesive stamp was put forward by Sir Rowland Hill, an English teacher, inventor and social reformer who campaigned for a comprehensive reform of the postal system, based on the concept of the Uniform Penny Post.

The Penny Black
Conservators at The Postal Museum prepare to transport abroad one of the only surviving sheets of the world's first stamp, the Penny Black, along with a sheet of Two Penny Blue's for the first time, in a specially designed, bomb-proof briefcaseMiles Willis/ Getty Images for The Postal Museum
The Penny Black
Detail of the missing top right-hand stamp traditionally collected by King George V from all sheets of stamps in history, including from this priceless sheet of the world's first stamps, the Penny Black, at The Postal MuseumMiles Willis/ Getty Images for The Postal Museum
The Penny Black
One of the only surviving sheets of the world's first stamps, the Penny BlackMiles Willis/ Getty Images for The Postal Museum

In 1837, British postal rates were high and complicated. Rowland Hill proposed an alternative idea, an adhesive stamp to indicate pre-payment of postage, in other words "...a bit of paper just large enough to bear the stamp, and covered at the back with a glutinous wash".

At the time it was normal for the recipient to pay postage on delivery, charged by the sheet and distance travelled. By contrast, the Penny Black allowed letters of up to 14g to be delivered at a flat rate of one penny, regardless of distance.

The Penny Black
Sir Rowland Hill (1795 - 1879) teacher, inventer and originator of the Penny PostMaull & Polyblank/ Getty Images
The Penny Black
Senior Conservator, Jackie Coppen, inspects a priceless sheet of the world's first stamps, the Penny Black, prior to them leaving the UK for the first timeMiles Willis/ Getty Images for The Postal Museum
The Penny Black
Conservator, Barbara Borghese, inspects a priceless sheet of the world's first stamps, the Penny Black, prior to them leaving the UK for the first time at The Postal Museum at The Postal MuseumMiles Willis/ Getty Images for The Postal Museum
The Penny Black
Conservator at The Postal Museum, Barbara Borghese, prepares to transport one of the only surviving sheets of the world's first stamp, the Penny Black, abroad for the first time in a specially designed, bomb-proof briefcase at The Postal MuseumMiles Willis/ Getty Images for The Postal Museum

Featuring a portrait of Queen Victoria, engraved by Charles Heath and his son Frederick, the design was based on a sketch provided by Henry Corbould. Corbould had taken inspiration from the 1834 cameo-like head by William Wyon, which was used to commemorate the Queen's visit to London 1837. For security reasons, the background of the stamp was of white-line machine engraving, also used on banknotes. It was supplied by the printers Perkins Bacon and was printed on the "D" cylinder press.

The Penny Black
A priceless sheet of Penny Blacks - the world's first ever postage stamp - is inspected prior to heading abroad for the first time in a specially designed, bomb-proof briefcase at The Postal MuseumMiles Willis/ Getty Images for The Postal Museum
The Penny Black
The Jacob Perkins' press, which printed the Penny Black and the 2d BlueBritish Library Hallway
The Penny Black
Senior Conservator, Jackie Coppen, inspects a priceless sheet of the world's first stamps, the Penny Black, prior to them leaving the UK for the first time at The Postal MuseumMiles Willis/ Getty Images for The Postal Museum
The Penny Black
One of the only surviving sheets of the world's first stamps, the Penny Black, is inspected prior to leaving the UK for the first timeMiles Willis/ Getty Images for The Postal Museum

Queen Victoria remained on British stamps until her death in 1901. Since then, British stamps bear a portrait or silhouette of the monarch, in some way or another. British stamps are the only stamps in the world where there is no indication of a country of origin – the monarch's image symbolises the United Kingdom.

The Penny Black
Senior Conservator, Jackie Coppen, inspects a priceless sheet of the world's first stamps, the Penny Black, prior to them leaving the UK for the first timeMiles Willis/ Getty Images for The Postal Museum
The Penny Black
Conservators at The Postal Museum prepare to transport abroad one of the only surviving sheets of the world's first stamp, the Penny Black, for the first time in a specially designed, bomb-proof briefcaseMiles Willis/ Getty Images for The Postal Museum