Many of those who believed this theory were supporters of the far-right U.K. Independence Party, better known as UKIP. They were convinced that the spy agency MI5 would try to change people's votes by erasing the penciled marks on their ballots. Some took to social media to warn voters, using the #usepens hashtag to get their message across. One Twitter user wrote, "Make sure you take a black pen with you to vote as pencil votes can be tampered with."
Others took it upon themselves to write messages on signs outside polling sites. This Twitter user took a picture of one that had "use a pen to vote" scribbled on it.
Others went as far as to interview the manager of the company that supplied the pencils to the polling booths. According to Jonathan Eley of the Financial Times, the manager of the company said that "there is nothing special about the pencils it supplies." Other people mocked the conspiracy theorists on social media.
The conspiracy behind pens vs. pencils dates to the 2014 referendum on Scottish independence, the Financial Times reports, in which people in Scotland voted on whether to remain in the United Kingdom.