EU citizens living in the UK have told of their anger after they were unable to vote in the European elections, ONA reports citing BBC.
The Electoral Commission said the "very short notice" from the government about the UK's participation in the elections had an impact on the process.
EU citizens can vote in the country they live in by registering to vote and completing a UC1 form.
But many complained of receiving the form late and of it not being processed by their local authority in time.
A campaign group said thousands of people have been affected by the issue.
The prime minister's spokesman recognised that there was "frustration".
The government announced on 7 May that the UK would be taking part in the European elections, having previously hoped that a Brexit deal would have been agreed by then.