Illegal workers were employed at a four-star London hotel run by one of Britain's wealthiest families.
An undercover investigation by BBC London found Romanian cleaners posing as Portuguese nationals.
A cleaner at the Kensington Close Hotel said she paid £190 for forged papers to cheat a ban on unskilled workers.
The hotel's owners - the Cola family, reportedly worth £158m - denied any wrongdoing, and said it has a contract firm that appoints cleaning staff.
Unlike other EU nationals, low-skilled Romanians do not have free access to the UK job market.
You know, I'm Romanian but here I don't have my documents. I have Portuguese [documents]. They are not mine
Illegal Romanian chambermaid
The government announced three weeks ago it would continue these restrictions in the current economic downturn.
To get around this, Romanians are paying criminal forgers to give them Portuguese and other EU identity cards.
The 550-room Kensington Close Hotel and Spa is owned by the Cola family whose fortune was estimated by the Sunday Times last year at £158m.
The hotel has said sole responsibility for checking workers' identities lies with a company called Central Hotel Services Ltd, which is contracted to employ cleaners on the hotel's behalf.
Central Hotel Services claimed it had "complied with the UK Border Agency's procedure for checking documents", and had recently dismissed four workers who failed these checks.
'Choose a name'
They hotel agency told BBC London that it had been the "victim of a scam" with Romanian workers presenting them with forged Portuguese ID cards.
BBC London sent a Polish undercover reporter into the hotel to work as a chambermaid, where she secretly recorded conversations with several Romanian cleaners.
One cleaner told her: "You know, I'm Romanian but here I don't have my documents. I have Portuguese [documents]. They are not mine."
Another Romanian cleaner claimed she knew of about 30 illegal cleaners, many of whom had bought their false IDs from the same forger, for £190 each.
She described how she had to provide a photo and choose a Portuguese sounding name as well as the cash.
During her week's work, the BBC London reporter also filmed staff meetings for chambermaids conducted in Romanian.
In a statement, the hotel said if illegal workers had been employed, then it was "entirely without the knowledge of the hotel and is solely the responsibility of Central Hotel Services."
It said: "Central Hotel Services Limited has a statutory and contractual obligation to ensure that their employees have the necessary permits to work legally in the UK."
The hotel said it was now taking the matter up with the job agency and reviewing its arrangements with them.
In a statement to BBC London, Central Hotel Services said: "Central Hotel Services Limited takes UK immigration law very seriously.
'Victim of a scam'
"We have been a victim of a scam where a small number of Romanians pretending to be Portuguese, French or Italian citizens, have presented us with forged identity documents to demonstrate their entitlement to work in the UK.
"We recently became aware of this issue at the Kensington Close Hotel and immediately launched an investigation.
"We are confident that we fully comply with UK immigration law and Home Office guidance, we carefully check and file employees' identity documents, but we are not able to identify accurate forgeries.
"The Home Office does not expect us to be able to do this.
"At our request, Home Office experts are now working with us to review the identity documents for all our employees. This process is ongoing. Any illegal workers will be dealt with appropriately."
The UK Border Agency said: "It is currently under investigation and we are working with both the hotel and the agency supplier to rectify this particular problem," a spokesman said.
The agency also said that it was targeting the criminals behind forged ID cards.
The spokesman added: "These are forgery factories. These are racketeers who are making money out of this business and exploiting these illegal workers.
"We have police detectives working with immigration detectives specifically to target the criminal gangs that are producing these documents."
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