The detailed evidence is being assessed by Parliament's Brexit Steering Group, chaired by Guy Verhofstadt.
The information was passed to the six members of the influential group by Polish EPP MEP Danuta Hübner, chair of Parliament's Constitutional Affairs Committee in the last term.
Despite warnings prior to the 23-26 May election, many EU citizens may have fallen foul of what has been called “cumbersome registration procedures” which required them to complete a UC1 form confirming they were not voting in any other Member State.
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The documentation sent to the Brexit group includes material said to demonstrate that the UK government failed to implement reforms to the voter registration procedure following a similar “mass disenfranchisement” of EU citizens at the previous European elections in 2014.
It also contains material suggesting that Theresa May’s government “made no effort” to prevent the UK's Electoral Commission from “undermining” civil society efforts to provide online voter registration facilities to continental EU nationals in the UK.
It is also claimed that the UK government allowed local authorities to use the “slowest possible form of communication” (second-class post) to provide relevant voter registration forms for continental EU nationals in the UK to fill out.
It is argued that the government could have decided to make funding available to local authorities to use first-class post.
"The government's failure is particularly galling because we were promised reforms to the registration process in 2014 but these have not materialised. Instead, the scale of the problem has got worse as a direct result of government inaction" Roger Casale, New Europeans
The document being assessed by the Brexit group also contains material “proving that the UK government failed to properly inform” continental EU nationals in the UK as to what they needed to do in order to exercise their right to vote.
It is claimed the message from the UK government to EU citizens in the UK was “make sure you are registered to vote in your home country”.
The alleged lack of sufficient time given by the UK government to continental EU nationals in the UK was potentially a violation of European law, it is further claimed.
The document was researched and compiled by the citizens’ rights campaign group New Europeans.
Roger Casale, of New Europeans, said, “The revelations are a major embarrassment for the Conservative government as they demonstrate quite clearly that it could have easily taken steps to avoid so many EU citizens being disenfranchised - but deliberately did not do so.”
“The fact that over one million EU citizens were, in effect, denied their right to vote, almost certainly had a major material impact on the results of the election.”
The former Labour MP noted, "The government's failure is particularly galling because we were promised reforms to the registration process in 2014 but these have not materialised. Instead the scale of the problem has got worse as a direct result of government inaction.”
“An independent inquiry is needed because it is the government itself that is in the dock.”
He continued, “Very large numbers of EU nationals were actually turned away from polling stations. It is insulting to tell EU citizens to go home and vote - the UK is their home. EU citizens have the right under EU law to participate in EU elections in the UK - the government has, de facto, taken that opportunity away from most of them.”
“And yet in a democracy, it should be the voters who choose their representatives, not the government who choose the voters,” Casale said.
Meanwhile, Scottish MPs, including First Minister Nicola Sturgeon, and MEPS have backed calls for an inquiry into what has been called the “vote denied scandal”.
New Europeans claims most EU citizens in Scotland may have been denied a vote in the elections. In Edinburgh, for example, only 20 percent of forms were returned before the 7 May deadline.
If repeated across Scotland, it could mean that as many as 100,000 EU citizens were denied the vote, said the group.
The information on registration figures came in response to a request from New Europeans to the Electoral Registration Office at the Lothian Valuation Joint Board.
Brian Brown, from the Office, said, "Our office is responsible for the compilation and maintenance of the electoral register for the four Lothian base Councils - the city of Edinburgh, East Lothian, Midlothian and West Lothian.”
“I do not currently have the figures broken down for each of the four separate areas. However, I can confirm that we issued around 40,000 UC1/EC6 forms to registered EU electors and around 8,000 of these were returned to us before the 7 May deadline."