Thousands of people ineligible to vote in the EU referendum have received ballot papers, the Government has confirmed, while also claiming the problem has been "cured".
EU nationals and a 17-year-old girl were among those sent postal voting papers despite not being allowed to take part in the June 23 poll, MPs heard.
Cabinet Office minister Oliver Letwin said it was believed around 5,000 papers had been wrongly sent out across the UK. And he added the Government needed to ensure the problems were not repeated at future elections.
Letwin confirmed the scale of the problem as the Commons debated emergency legislation to extend the deadline to register to vote in the EU referendum to the end of Thursday.
The Government registration website crashed close to the original deadline of midnight on Tuesday, resulting in potentially thousands of people being unable to submit their applications.
Speaking during the debate, Conservative MP Henry Smith (Crawley) told Letwin: "In terms of the checking of those who are eligible to vote, with large numbers seeking to be on the electoral roll, I from my constituency have had a number of reports of EU nationals being sent postal voting papers and also, just last night, somebody calling me to say their daughter who was 17-years-old had received voting papers.
"What sort of assistance will be provided to electoral services officers and returning officers to ensure the vote is secure in that sense?"
Letwin, in his reply, said: "There has been, in a few cases, a problem with the issue of votes to people who were not eligible for voting.
"That problem has now been inspected and cured, and we need to make sure that in future elections it doesn't happen."
Conservative former cabinet minister Liam Fox, a leading Leave campaigner, asked Letwin: "You say this problem of ballot papers being issued to those not eligible to take part in this election has been identified and cured.
"Can you therefore give us an idea of the scale of the problem? How many of these wrong ballot papers were issued?"
Letwin replied: "We believe it to be around 5,000. Nationally."
Earlier, Letwin said the voter registration website had been doubled in capacity after noting it was "not surprising that it fell over" during the surge.
He added the Government believed there was "no untoward event" which led to the problems, and there were 214,000 applications to register to vote in the hour before the system crashed.
Letwin said: "What we cannot tell, because it's the nature of the computer system not to be able to tell us, is how many people either did try or would have tried during the succeeding period of 90 minutes or so when they were unable to apply."