Users have until 11.59pm BST to register to be able to cast their votes on 23 June.
According to the Electoral Commission, 226,000 applications were received on Monday, the second highest total for one day since the introduction of electronic registration in 2014. Over 1.65m people had applied for a vote since a campaign began last month.
Trying to help my 86 year old mum register to vote #EUReferendum & getting a '502 Bad Gateway' message every time. That's her vote lost— Kieran Martin (@creativekieran) June 7, 2016