Leave voters have been urging fellow supporters on social media to bring pens with them to mark their ballots.
The hashtags #usepens and #pengate were circulating on Twitter encouraging pro-Leave voters to come to UK polling stations armed with pens over suspicions that their pencilled-in crosses could be tampered with.
In response, renowned physicist Professor Brian Cox quipped: “I voted in pencil just in case MI5 need to change it later.”
Frustrated voters claimed online they had been told they could not use a pen in various polling stations - despite both ink or graphite being acceptable voting tools in the eyes of the Electoral Commission.
East Northamptonshire Council has urged voters not to use pens when they cross their EU referendum ballot papers as it could cause them to “smudge”.
Mid-way through polling day, the authority tweeted: “Please use a pencil on your ballot paper as ink can smudge when its folded which could make the paper doubtful & it may not be counted.”
In the UK, pencils are traditionally used for marking ballot papers and available inside polling stations for voters to use, a spokesman for the Electoral Commission said.
“However, there is nothing to stop a voter from using a pen to mark their vote - there is no legal requirement for ballot papers to be marked with a pencil,” he continued.
“The reason that pencils are traditionally used is partly for historical and practical reasons: with ink pens there is always a risk that they may dry out or spill.
“Also, ink may cause some transfer of the mark the voter has made on the ballot paper when they fold it, thus potentially leading to a rejection as it may look like they have voted for more options than they are entitled to.”