The number of staff the Home Office intends to recruit to register the 3 million EU nationals in Britain in the run-up to Brexit means each caseworker will have to take 100 decisions a day, an official watchdog has warned.

David Bolt, the chief inspector of borders and immigration, told MPs on Tuesday that an extra 500 staff being recruited to register EU nationals already living in Britain would face severe pressure. “That’s a lot of decisions, even with a light touch,” he told them.

The Home Office has said an extra 700 staff had been recruited to deal with European immigration casework and intended to recruit a further 500 staff to register the 3 million EU nationals by next April.

Registration is expected to start next summer and will take place over two years. The immigration minister, Brandon Lewis, told MPs last week that the extra 1,200 staff would be used only partly to register EU nationals.

When Yvette Cooper, the chair of the House of Commons home affairs select committee, asked whether it was realistic for an immigration decision-maker to deal with 100 cases a day, the chief inspector replied: “I would not want to be doing it myself.”

He told MPs the Home Office intended to introduce a streamlined system “which was not intended to be like any casework system that any of us have seen” in which the default position would be to grant “settled status” to EU nationals who have been in Britain for five years.

“My concern is if the intention is, in all but a tiny proportion of cases, the onus is on the department to grant. If it is automated as far as possible and you click the button to grant then you could do it, but as soon as you have one that is more complex then it all gets thrown out. I am not sure how it is going to be dealt with.”

His concern was shared by Adrian Berry, the chairman of the Immigration Law Practitioners’ Association, who told the MPs that the prospect of each caseworker taking 100 decisions a day was “for the birds”. He said it would only work if every applicant had five years’ worth of P60 earnings forms proving that they meet the residency requirement.

“This is not realistic,” he said.