He showed his 2.4 million Twitter followers a video in which experts were able to change the number of votes for each candidate in the computer memory – and also alter the paper-trail backup.Security experts were easily able to hack into a Sequoia AVC Edge voting machine and change results

He said hacking individual machines would be easy, but election officials would probably be able to detect fraud after polling day – which could be crucial if the result is contested.

Snowden said: "Researchers just demonstrated how to hack the official vote count with a $30 card.

"Little time to patch this vuln(erability), but can still forbid use of this model, run statistical analysis after polls close on rest to ID outliers."

He added: "Hacking voting machines: not that difficult. Hiding a secret deviation in votes from after-the-fact statistical analysis: nearly impossible."