The European Commission has complained to the UK government over obstacles it says stopped EU citizens entitled to vote from casting a ballot in last month's European elections.
A letter sent to the Cabinet Office claims that voters from other EU states - who are entitled for vote in EU wide polls regardless of where they live in the union - faced "a number of obstacles to participation".
The letter from EU Justice Commissioner Vera Jourova , which has been seen by the BBC, said the UK had an "obligation to respect the right to vote of EU citizens and to take the measures necessary to ensure such voting rights can be effectively exercised".
Last month an unknown number of EU citizens were turned away from polling stations, with the hashtag #deniedmyvote trending on Twitter as votes were counted.
In addition to registering to vote, EU citizens had return a UC1 form declaring they would not vote in another EU state.
But the letter claims many voters requested the form but it did not arrive, while others submitted forms that were never processed.
Last night Cat Smith MP, Labour ’s Shadow Minister for Voter Engagement, called for an investigation into the election - which was organised at short notice after the government failed to secure an EU withdrawal agreement and had to delay Brexit until October 31.
She said: “Labour repeatedly warned the government that European citizens living in the UK would be denied their right to vote because of its incompetent approach to Brexit.
“Following this intervention from the EU Commission and the chaos witnessed at the polls, an investigation into this scandal is needed as a matter of urgency."
Mrs Jourova's letter asked the UK government to provide information about the obstacles that EU voters had faced in the UK, as well as data "on the incidents reported and the complaints lodged with the UK authorities".
The UK hosts about 3.2m nationals from the other 27 member states, while about 1.3m UK citizens live elsewhere in the EU.
In a statement on Ms Jourova's letter, the UK Cabinet Office told the BBC: "The UK Government took all the legal steps necessary to prepare for the European Parliament elections, including putting in place all the necessary legislative and funding elements to enable Returning Officers to make their preparations."
