1) The office carried out secret interceptions

The main role of the Secret Office was to intercept mail between Britain and overseas, and to read it. Foreign post and official dispatches passed between Britain and the rest of the world via the Packet Service: a fleet of fast ships sailing regular routes. Foreign mail bags were sent to the office, where on their arrival teams of translators and decipherers read through the contents to copy out any relevant information in English.

The copies were then sent on to the secretary of state, and the mail was returned to the GPO for delivery as normal. From the 1790s, mail arrived at the office twice a day: at 10am and 2pm. In some cases, the inspectors could be given as little as half an hour to read through all the items and send them on their way again.

Secrecy was naturally at the heart of these operations. If foreign governments realised their mail was being read, they could instead send it by special messenger, denying Britain access to valuable intelligence. Located near the Foreign Post Office, the Secret Office was so well concealed that employees of other GPO departments were completely unaware of its existence.

During the second half of the 18th century, it was the role of the chief clerk to examine any letters that he thought might be useful. However, inspections of certain items could also be commanded by the king. In 1755, for instance, King George II specially requested that the French mail bags be inspected for letters from a ‘Mr Barry’.

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2) It wasn’t your average day job

A career in the Secret Office certainly wasn’t run of the mill. At the heart of its operations was a team of decipherers, which in 1748 included a ‘Chief Decypherer’ and Second, Third and Fourth Decipherers.

These positions were lucratively paid, with the head of the group earning £1,000, and his subordinates from £800 to £100. Considering the average wage for a mail ship crewman was around a shilling a day, or £18 and five shillings per year, these pay-outs probably provided a strong financial motivation to keep the office under wraps. Even the ‘Door Keeper’ received £50 per year. Other employees included a chief clerk, general clerks, and an intriguingly-named ‘Alphabet Keeper’.

3) Staff levels reflected the state of warfare

Naturally, when Britain was at war there was a far greater need to monitor communications for possibly valuable information. In 1752 the office employed five people, but by the time the American War of Independence was in full flow in 1776, there were 10.

Numbers remained high with the outbreak of the Napoleonic Wars between Britain and France, beginning in 1803 and lasting through to 1815. In 1816, a year after peace, staff numbers in the office were reduced to six.

In addition to more people being employed for the Secret Office, the number of Packet ships running between Britain and overseas also increased dramatically during times of war. At the height of the Napoleonic Wars, there were around 40 ships sailing, carrying soldiers’ mail as well as government dispatches. The Packets also smuggled newspapers out of France and spies into it.

4) Its exposure led to the state spying scandal of its day

During the 1840s, the Secret Office was somehow exposed and an inquiry was held to investigate its activities. Although the interception of foreign mail was not seen as giving cause for concern, there were many questions raised about the possible examination of domestic mail. Spying on others was accepted, but spying on one’s own country was distinctly scandalous.

Though every attempt was made to neutralise the issue, the GPO tentatively admitted that British letters had been targeted. In one statement, it was said that the chief of the ‘Secret Department’ never desired to read domestic mail, and that “inspection of private correspondence is altogether and entirely disclaimed”. That was, “excepting in a very, very few cases, when the task has been imposed upon him by an Authority which he was bound to submit to.”

The desire to crack down on treasonous activities seems to have outweighed any sense of loyalty between countrymen, with the GPO arguing that they “did not consider it to be the duty of Government to facilitate and protect the conveyance of Treason from one end of the country to the other.”

The 1840s enquiry would seem to mark the end of the institution's activities, but the secretive nature of the office makes it impossible to be sure.

5) It’s impossible to know how successful it was

Because of the highly secret nature of the office – it was referred to in 1782 as ‘Our Secret Service’ – facts and figures are hard to come by. Warrants requesting that items of correspondence be sent to the office were produced, and these inspections certainly led to arrests in Britain, but for the majority of the time there was no official practice for recording the warrants. In fact, they were frequently burned after being received by the postmaster general.

Warrants commanding the interception of foreign mail tended to lead to the copying out of passages, whereas ‘criminal’ warrants relating to domestic mail often simply permitted its seizure. In 1758, Dr Florence Hensey was convicted of high treason and sentenced to death, based on ‘treasonous correspondence’ seized by the office.

Hensey had a friend in France whom he corresponded with and later transmitted intelligence to, for the sum of £25 a month. In an attempt to outwit any other readers, Hensey had written in lemon juice between the lines of a seemingly innocent letter.

In another case, a letter home by a sailor ‘pressed’ – forcibly conscripted – into the Navy was seized during the Napoleonic Wars. Writing to his wife, the sailor bemoaned his treatment and outlined a plan to escape, little knowing that the letter would be read and kept as evidence against him.

There is no way of knowing for sure how many arrests or strategic decisions were made based on intercepted mail. Nevertheless, the longevity of the Secret Office, its actions shrouded in mystery for 200 years, suggests that for a time it was a valuable extension of the power of the British government.