Resolving mistakes made when new rules come into force to freeze or close the bank accounts of illegal immigrants could be an "absolute nightmare", a former board member of TSB has warned.
Banks and building societies will carry out checks from January to see if account holders are legally in the UK.
MPs backed the move in 2016, but Philip Augar said mistakes would be made and "poor individuals" would suffer.
The Home Office said the system was "fair" but "firm".
Financial institutions will be provided with a list from anti-fraud organisation Cifas on people who are liable for removal or deportation from the UK or who have absconded from immigration control.
The organisations will then have to report any names they discover and freeze or close the accounts.
According to the Guardian, 70 million accounts will be looked at quarterly to check the immigration status of the holders.
Mr Augar, who also previously worked at the Home Office, said it was reasonable for "law breakers to expect the full weight of enforcement", but raised concerns about how mistakes would be addressed.
He told BBC Radio 4's Today programme: "This is in the hands of the Home Office and the banks, neither of which are exactly known for flawless execution.
"What happens when something goes wrong - if a bank account is frozen and the owner is entirely legal or if they've simply got the wrong name?
"Resolving those situations will be, in some cases, smoothly done, but I can imagine it being an absolute nightmare for some poor individuals.
"The problem is going to be multiple names will come up, the wrong names and bank accounts will be frozen, and it is how this is resolved, that will be the difficulty."
The checks form part of a series of measures in the Immigration Act 2016 aimed at encouraging illegal immigrants to leave the UK voluntarily.
It follows on from rules passed in 2014 intended to check the immigration status of anyone opening a new bank or building society account.
The Home Office said it would only share details of immigrants who were in the UK unlawfully.
This would include those who had exhausted all appeal rights and who should be denied access to banking services, Home Secretary Amber Rudd said.
Ms Rudd added migrants in the UK legally would "of course" be able to continue using their accounts.
The Joint Council for the Welfare of Immigrants criticised the action saying migrants "with every right to be here" will be affected.
Chief executive Satbir Singh told the Guardian: "The government's own record shows that it cannot be trusted even to implement this system properly.
"Immigration status is very complex, and the Home Office consistently gives out incorrect information and guidance."
A Home Office spokesman said: "We are developing an immigration system which is fair to people who are here legally, but firm with those who break the rules.
"Everyone in society can play their part in tackling illegal migration."
The plans also include a crackdown on landlords and employers if either knowingly provide a service to anyone who has no right to be in the UK.