EU citizens living in the UK raised concerns that they cannot board flights into the country starting from 1 October.
This came after the UK government has approved new rules over ID cards and passports.
According to the rules, EU citizens who do not have the post-Brexit right to live in the UK will not be able to use EU, EEA or Swiss national ID cards to enter the country.
It means that EU citizens not settled in the UK arriving for short visits or visa stays will be required to show a passport under post-Brexit immigration rules.
Those settled in the UK can continue to use ID cards until 2025.
Brussels fears that airlines will struggle to make the distinction at boarding gates. Thus, the UK has not issued EU citizens living in the UK since Brexit with physical residency documents.
Instead, airlines will have to log into a government website. Check a digital code the passenger can also generate on their phone or laptop to prove their status.
Earlier, the UK had promised that EU citizens who applied to stay in the country would have their rights protected.
However, the Home Office has come under fire for willingly attempting to deport EU citizens.
Bail for Immigration Detainees (Bid) said that unless a meaningful response is received from Home Office this week, the body will file a complaint in European courts. It had raised concerns over the Home Office failure to protest the rights of the citizens.
Lawyers have launched legal battlers to protect the EU nationals following the Brexit deal.
The numbers affected could be significant.