"We fear that by the application deadline next year neither the Home Office nor organisations like the3million will know how many EU citizens will not have secured their rightful status."

The report also raises concern that the global Covid-19 pandemic has led to a reduction in outreach capacity to vulnerable EU citizens most likely to be among those who fail to apply to the scheme, and the disruption of official data collection on which EU citizen population estimates rely.

Tory MP admits EU settlement scheme sometimes 'doesn't work as well as we expect'

Ms Sumption added: “While the EU settlement scheme is in some respects less exposed to the coronavirus crisis than other parts of the immigration system that rely more heavily on face-to-face contact, COVID-19 nonetheless has important implications for the scheme.

“That includes lower capacity for supporting vulnerable EU citizens, including those most likely to be poorly measured in official statistics. It also remains to be seen how much the quality of the data on EU citizens in the UK will suffer as a result of the pandemic.”

A Home Office spokesperson said: “More than 3 million grants of status under the scheme have already been made and there is still more than a year left to apply until the deadline of 30 June 2021.

“We are working closely with employers, local authorities and charities to raise awareness of the EU settlement scheme and identify those who are eligible. A wide range of support has been available for applicants since the scheme opened, including throughout the coronavirus pandemic.”