Data published by the Electoral Commission on the 2016 EU referendum and the 2017 UK General Election show that thousands of postal votes were rejected, not received, or lost.
In the 2016 EU referendum, 167,392 postal votes were rejected and not included in the final count. 25,656 postal ballots were identified as ‘lost’ or ‘not received in time for another to be issued.’ Postal voters are not informed whether or not their ballot was received or counted.
In the 2017 UK General Election, more than 18,000 ballot papers appear to have been accidentally spoilt with voters ticking more than one candidate. 24,081 postal voters were lost or not received in time for another to be issued. 168,734 postal votes were rejected in this election. More than 16,000 postal votes were returned the day after the election. In the 2015 General Election, an estimated 27,500 ballots were accidentally spoilt.
Reasons for postal votes being rejected include signatures and dates of birth being missing, or mismatching.
In addition to postal votes being rejected, more than 10,000 people tried to vote on polling day and were found to not be registered. The same data appears to be unavailable for the 2016 EU referendum.
The Electoral Commission data can be accessed here.