Why has this happened?

EU nationals have the right to vote in European parliament elections in the countries of which they are citizens.

If they want to vote while living in another EU country they must transfer their vote as well as registering in the normal way, the Electoral Commission (EC) has said.

European law requires this measure, and UK law says the forms involved – which should be sent out automatically by councils – must be returned 12 working days before polls open.

The EC said that many people in the UK may not have known about it in time.

"The very short notice from the government of the UK’s participation in these elections impacted on the time available for awareness of this process amongst citizens, and for citizens to complete the process," it said in a statement released on Thursday.

It added: "This legal process could be made easier for citizens, and the Commission made the case for doing so following the last EU elections in 2014.

"However, improvements to the process are reliant on changes to electoral law, which can only be taken forward by government and parliament."