Pontins kept a list of Irish surnames in a bid to stop Gypsies and Travellers from staying at its holiday parks, a whistleblower revealed.
Staff were required to block potential customers with those surnames from booking holidays as part of the firm's efforts to keep out "undesirable guests".
Staff also monitored calls and refused or cancelled bookings made by people with an Irish accent, it was revealed.
The Equality and Human Rights Commission (EHRC) verified the claims and found that Pontins discriminated on the basis of race and breached equality laws by declining to provide its services to guests of a certain race or ethnic group.
Downing Street condemned Pontins' use of a blacklist as "completely unacceptable".
Were you a potential guest who was affected by the policy? Email webnews@mirror.co.uk.
Pontins' owner has entered into a legal agreement with the human rights watchdog after whistleblower's claims were revealed.
Britannia Jinky Jersey Limited is working with EHRC to prevent racial discrimination after a Pontins employee said the holiday park company was operating a discriminatory bookings policy.
The EHRC verified the claims and said practices included a list of Irish surnames published on its intranet page, with staff required to block potential customers with those names from booking.
It said staff monitored calls and refused or cancelled bookings made by people with an Irish accent or surname, and Pontins' commercial vehicle policy excluded Gypsies and Travellers from its holiday parks.
By declining to provide its services to guests of a certain race or ethnic group, Pontins was "directly discriminating on the basis of race" and breached the 2010 Equality Act, the EHRC said.
Britannia Jinky Jersey Limited said it was working to enhance its staff training and procedures to further promote equality throughout the business.
Alastair Pringle, EHRC executive director, said: "It is hard not to draw comparisons with an undesirable guests list and the signs displayed in hotel windows 50 years ago, explicitly barring Irish people and black people.
"Banning people from services based on their race is discrimination and is unlawful. To say that such policies are outdated is an understatement.
"It is right to challenge such practices and any business that believes this is acceptable should think again before they find themselves facing legal action.
"We will continue to work with Pontins and Britannia Jinky Jersey to ensure that our agreement is adhered to and its practices improve."
A spokesperson from Britannia Jinky Jersey said: "Britannia Jinky Jersey Limited has agreed to work together with the Equality and Human Rights Commission to further enhance its staff training and procedures in order to further promote equality throughout its business."
As part of the agreement, Pontins must investigate the "undesirable guests" list, take appropriate action and ensure lessons are learned.
It must commission a review into its booking and commercial vehicle policy and consider any recommendations, and provide equality and diversity training for staff each year.
If it does not adhere to these terms, the EHRC can launch a full investigation.
Downing Street condemned Pontins' use of a blacklist of "undesirable guests".
The Prime Minister's official spokesman said: "This is completely unacceptable.
"No-one in the UK should be discriminated against because of their race or ethnicity.
"It's right that the Equality and Human Rights Commission and Pontins investigate and address this."
