Spain has seen COVID-19 cases rise in the last few weeks, and Britain announced late on Saturday it was taking the country off a safe-travel list.
The new rules took hold at midnight Saturday, hours after being announced, causing uncertainty for holidaymakers and leading to criticism from travel industry leaders.
This decision of sudden implementation was not well-received by the travellers.
British Foreign Secretary Dominic Raab defended the quarantine decision on Sunday, as a "real-time response" to a jump in infections in Spain.
The opposition has, however, called this move by the Boris Johnson administration to be "frankly shambolic".
Britain has also advised against all but essential travel to mainland Spain, leaving the islands out of the advice but including them in the quarantine measure.
The Joint Biosecurity Centre and Public Health England monitor a number of different factors to determine a country's quarantine status, including the number of cases in European countries and the trajectory of cases rising - as well as the geographic prevalence of the spread within the country.
Travellers arriving in Northern Ireland from Spain and its islands will have to quarantine for 14 days, the Department of Health has confirmed.
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TUI, Europe's biggest holiday company, said on Sunday it was canceling all holidays from the United Kingdom to mainland Spain up to and including August 9 following updated travel advice by Britain imposing a 2-week quarantine on those returning from the Iberian country.
It said the move was "throwing thousands of Britons' travel plans into chaos", reports the bbc. Last year, British holidaymakers made up 20% of all arrivals to Spain, higher than any other nation.
And the British move will affect not just Spain's tourism sector but airlines and travel companies struggling to get back to business.
"Tui UK have taken the decision to cancel all holidays to mainland Spain up to and including Sunday 9 August 2020", the company said in a statement.
Criticising the new measures, British Airways said it was "disappointed", but it said its flights would continuing to operate.
A spokesperson from the Spanish foreign ministry said that the country respected Britain's decision, while stating that Spain remains safe, with only localized, isolated and controlled outbreaks.
On Friday Spanish Foreign Minister Arancha Gonzalez Laya told CNN that like many countries around the world that have managed to control the disease, Spain "has outbreaks but the governments - both national and regional - are working to isolate cases as soon as they appear".
Britain itself has been the worst hit country in Europe by the pandemic, with more than 328,000 cases and an official death toll of more than 45,600.