The oldest tea in Britain, a box of leaves and flowers neatly labelled “a sort of tea from China” more than 300 years ago, has turned up in the stores of the Natural History Museum in London.

When it was brought back by James Cuninghame, a Scottish surgeon and amateur naturalist in the late 17th century, tea was still a fabulous rarity, sold at between six and 60 shillings a pound, 10 times the price of even the best coffee. Cuninghame was a passionate plant hunter, and may have collected his samples at Amoy in Fujian province, or on the island of Chusan where he described tea growing wild and the local farmers preparing the leaves for the drink.

Academics from Queen Mary University London uncovered his samples while researching the history of the exotic import which rapidly became the British national drink, for a book to be published this summer. They yearned to taste it but were not even allowed to touch the tea. Instead, the glass lid of the small cardboard box was lifted, and they were permitted to sniff the contents.

“It had a very very faint scent of hay,” one of the academics, Matthew Mauger, said. “In the 18th century, writers struggling to describe this exotic new drink do refer to the smell of hay,” his co-author Richard Coulton said. However, he added: “Fresh tea really doesn’t last very long – I doubt very much that it would be drinkable.”

Facebook Twitter Pinterest Matthew Mauger, left, and Richard Coulton, the lecturers who found the oldest tea in England. Photograph: Graham Turner

Cuninghame corresponded with an even more passionate and far wealthier medicine man and plant collector, Hans Sloane. Sloane had introduced England to another exotic brew. By adding milk to ground cocoa beans from Jamaica he reduced the bitterness, and gave birth to drinking chocolate. Like chocolate, tea was first promoted for its medicinal value.

Sloane acquired Cuninghame’s samples both of wild tea leaves on the branch, which were added to the thousands of pressed plants in his herbarium, and the small box of processed and prepared tea, which became item 857 in his “vegetable substances” collection. His collections grew so vast that they became the basis of both the British Museum and the Natural History Museum, which is where the tea ended up, rediscovered with its original 857 label only when the 18th-century catalogue was finally digitised.

“The very best teas, favoured by the emperor, were described as large leafed and loosely rolled, and that’s just what we see in this sample, so we like to think we have a box of imperial tea,” Coulton said.

The 300-year-old tea looked remarkably like a pack of leaf tea which a colleague had just brought them as a gift from China, which produces a pale gold tea with – undeniably – the scent of a new-mown hay field.

Their book traces the history of the craze for a drink which, visitors to England were soon noting in wonder, became a national obsession.

From the start despite, or because of, the enormous cost, tea drinking spread rapidly among the wealthy. By 1660 Samuel Pepys recorded: “I did send for a cup of tee (a China drink) of which I never had drank before.” The porcelain cups and tea pots came in with the leaves, used as ballast to counteract the problem for shippers of a very bulky but very light cargo.

The drink became cheaper in the 19th century when the East India Company began to import in bulk tea grown on land it controlled in India, instead of entering into fractious negotiations with Chinese merchants.

It was a tax reform which finally brought tea within the grasp of the ordinary worker, when in 1784 William Pitt the Younger slashed the tax on it from 120% to 12.5% – and whacked up the window tax instead – in an attempt to soothe the national epidemic of tea smuggling.

Coulton and Mauger traced cabinet records of discussions of how much tea drinking would increase, and tax evasion plummet, as the government sought to balance the books. By the end of the 18th century, they found that small quantities of tea were on the weekly shopping list for even poor labouring households.

Tea became an emblem of domestic virtue and temperance – though the temperance movement was initially suspicious of its dangerously stimulant qualities – and the tea break the joyful mandatory pause in an exhausting day’s industrial labour.

Guiltily, both Coulton and Mauger admit that, despite the impressive glass tea pot and array of exotic leaf teas in their shared office, their usual cuppa is a bleary-eyed reach for the nearest tea bag.

• This article was amended on 27 May 2015 because an earlier version said tea became cheaper in the 18th century when the East India Company began to import in bulk tea. This has been corrected to say 19th century.