The European Union (EU) fears millions of its nationals living in the UK will be left stranded in a legal no-mans land after the country leaves the bloc because of the weaknesses of the British immigration system, according to a leaked document.

Members of the European Parliament (MEP) and senior European diplomats fear that chaos will ensue as the Home Office does not have the information or systems in place to select who can stay, once the UK restricts access to nationals from the other 27 EU member states, the Guardian reported. 

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It is widely assumed that, at an early stage of article 50 — official process of exiting the EU — negotiations, Britain and the bloc will agree a cut-off date after which foreigners who have settled in the country will not have an automatic right to remain.

But the leaked document on Saturday, drawn up by MEPs on the European parliament's employment committee to aid the EU's Brexit negotiations, warned: "The UK has no population register. In practice it would be difficult to determine which EU27 citizens were residing legally in the UK before the Brexit would have taken effect.

"If all 3.3 million EU citizens were to initiate procedures aimed at proving the ‘exercise of treaty rights', the administrative system would be overburdened."

Former Prime Minister Tony Blair's government was forced to drop plans for a national identity card and population register in 2007 because of concerns over the security of citizens' data.

The Cabinet Office advises that EU nationals do not need to register for any documentation in order to enjoy free movement rights and responsibilities in the UK, reports the Guardian. 

The government does not track EU nationals as they enter and leave the country. The databases of the Department for Work and Pensions, Revenue and Customs and the Home Office do not share information.

EU nationals who have lived in the UK for five years can gain automatic permanent residency status, although there have been numerous cases of employers asking for their foreign employees to acquire permanent residency cards in order to guarantee their jobs.

There has been almost a 50 per cent increase in the number of EU citizens applying for permanent residency documentation since the vote on June 23.

EU nationals say that to obtain permanent residency cards they have to complete an 85-page form requiring huge files of documentation.

The European parliament is also due to debate the rights of EU nationals in both the UK and on the continent on March 1.