Nearly 50 traffic wardens have left their jobs in one London borough after immigration checks were carried out.
NCP Services Ltd took over the contract for wardens in Lambeth in August and examined the backgrounds of 150 staff.
Forty-eight of these then "resigned or just didn't turn up for work", the company's spokesman, Tim Cowan, said.
"We can't say for definite they didn't have the right to work in this country but we did see some passports and documents we were concerned about."
He told BBC London 94.9: "We certainly did feel there was an issue there.
"You have to be very careful and the Home Office has made it perfectly clear that it's the responsibility of the employer to make sure that the people they hire have the right to work in the UK."
And he added NCP Services Ltd used "sophisticated document-checking methods" to ensure staff were employed legally.
Businesses can be fined £10,000 for breaking the law by recruiting those without the correct visas or valid passports.
And the UK Borders Agency has warned it makes "no apology for taking tough action against employers who knowingly hire illegal workers, often exploiting them and undercutting the minimum wage".
The 150 staff were previously employed by Apcoa Parking and transferred to NCP Services Ltd when the new contract came into force.
Of these, 100 were wardens, or "civil enforcement officers" as their employer describes them.
Checks had begun on "day one", Mr Cowan said.
'Support' for checks
Asked to respond to the exodus of staff, an Apcoa spokesperson said: "We work closely with the Home Office and confirm that at every stage, Home Office guidelines for the prevention of illegal working have been followed.
"We are unable to comment any further as we no longer operate this particular contract."
Lambeth Council said in a statement that it absolutely supported NCP Services in its efforts "to ensure staff employed in Lambeth have the right to work in this country".
It added: "Lambeth Council is committed to a firm but fair parking system and by appointing NCP Services we are turning a new page in parking in Lambeth.
"We are investigating this, and it would be inappropriate for us to comment on the practices of the previous contractor at this stage."
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