An archive of thousands of undelivered personal letters from all over the world, seized from ships captured during Britain’s naval wars over three centuries, are to be digitised in a project offering an intimate glimpse into people’s lives.

The letters, found in mailbags, with many bearing wax seals and some still unopened, have so far yielded personal accounts, some heart-rending, and journals, sheet music, drawings, poems and a packet of 200-year-old seeds from South Africa.

In one letter, a priest from Lima is sending home to his mother the lace ribbon used in his ordination ceremony, a gift she would never receive.

Known as the Prize Papers, the 160,000 letters are from about 35,000 ships captured by the British navy or privateers, during wars of the 17th, 18th and 19th centuries. They give a unique insight into the development of the colonies, of the slave trade and international trade.

Most ships carried mail as well as the main cargo. The letters were submitted as court evidence before the Prize Court of the High Admiralty, which was near St Paul’s in London, as proof of there being an enemy ship, so that the captors could claim the spoils.

From the Prize Court, the letters, still in their mail sacks, were moved to the Tower of London. They stayed there until 1850, when they transferred to the Public Record Office in the City of London, where they remained, in about 4,000 boxes, uncatalogued because of a lack of funding,

The 20-year, €9.3m (£8.4m) Prize Papers Project, funded by Union of the German Academies of Sciences and Humanities and carried out in cooperation with the German Historical Institute London and the National Archives in Kew.

The collection includes missives in 19 languages, documenting the experiences of the public, traders and sailors during several wars – including the American and French revolutionary wars, and Napoleonic conflicts; there are letters from captured ships brought into ports in England, Ireland, Portugal and the Channel Islands during the 14 naval wars between 1652 and 1815.

Facebook Twitter Pinterest Dagmar Freist, a professor at Oldenburg University, looks through a mail bag from an American merchant ship that traded between New York and Bordeaux in 1812. Photograph: Kirsty O'Connor/PA

Items from the La Robushe ship include a register of 504 men, women and child slaves from the Gold Coast who were sold in Saint-Domingue – now Haiti – in 1756. Each buyer is listed, along with the number of slaves they bought and the credit terms for each purchase.

In another letter, Elizabeth Sprigs, an indentured servant, writes from Maryland in the US to her father in England in 1756, complaining of her working conditions and pleading for him to send clothes. “What we unfortunate English people suffer here is beyond the probability of you in England to conceive. Let it suffice that I one of the unhappy number, am toiling almost day and night, and very often in the horses druggery,” she wrote, adding she had “scarce anything but Indian corn and salt to eat”.

A ship’s master, from the German merchant ship Concordia, writes to his wife in English about running into English privateers who “rab mee from all mee fouls, and drunk all ouer wyn and Beer”.

Randolph Cock, a specialist at the National Archives in Kew, south-west London, who is working on the Prize Papers, is responsible for the painstaking job of sorting the boxes prior to the digitisation of the material. He swiftly discovered many were jumbled up.

“At some point, perhaps the shelves they were stored on fell down. They were scooped up into a big bag, and taken to the Tower of London. Then they were taken out again and put into boxes and taken to the Public Record Office. My job is to put it back into order, and find out which ship each letter came from,” he said. He estimates it will take another 11 years to complete the task.

Dagmar Freist, from Oldenburg University, is the project director for the Prize Papers. She secured funding from Göttingen Academy of Sciences and Humanities to digitise the collection for free online access. She described the collection as a “fascinating unarchived archive because nothing has been thrown away”.

She said: “It’s very hard to find material which allows you to look at how people interacted across borders, even across seas and continents. This archive is completely by chance, so we have documents that would not have survived, they would not have been in an archive, and that allows us to get a new perspective on global relations when European countries were starting to build colonies.

“It cuts across the nationally orientated empire history to see how people were engaging with social groups in other parts of the world as traders – and we have a lot of female traders who suddenly become visible in these documents – as well as those accompanying families, missionaries, slave owners and sailors.”

• This article was amended on 6 September 2018. The original incorrectly stated that the project was funded by Oldenburg University in Germany. The project is actually funded by the Union of the German Academies of Sciences and Humanities and is carried out in cooperation with the German Historical Institute London and the National Archives in Kew.