The announcement follows concern that tens of thousands of trips booked after the country was put on the UK Government's green travel list would be cancelled.
The uncertainty was caused by Portugal's ministers deciding on Thursday to continue its current level of coronavirus lockdown restrictions until May 30 at the earliest.
But tourist body Visit Portugal said the minister of state for foreign affairs has confirmed that UK tourists who have had a negative PCR test within the previous 72 hours will be welcomed from Monday.
UK holiday firms have experienced huge demand for trips to Portugal since the Scottish and UK governments confirmed holidaymakers would not be required to self-isolate on their return from visits to the country.
Scotland will permit foreign holidays from May 24.
EasyJet has added 105,000 extra seat to its flights serving green-tier destinations, while Tui plans to use aircraft which normally operate long-haul routes to accommodate the surge of people booked to fly to Portugal.
Paul Charles, chief executive of travel consultancy The PC Agency, said: "It's green for go from Monday.
"Overseas travel will restart as scheduled in the Prime Minister's road map and well done to Portugal for making it happen.
"Travel can be done safely and responsibly.
"We now need to see a wider green list from the start of June."
EasyJet said in a statement: "We welcome the decision from the Portuguese government meaning that those travelling from the UK will be allowed to travel safely to Portugal from Monday.
"We look forward to reuniting friends and family and taking customers on a long-awaited holiday this summer."
But Simon Cooper, chief executive of online travel agent On the Beach, said the situation "illustrates precisely why now is not the time to encourage new holiday bookings for this summer".
The company stopped selling summer holidays on Wednesday due to a lack of "certainty or clarity" in relation to travel rules.
Joanne Dooey, president of the Scottish Passenger Agents’ Association (SPAA), had earlier said travel agents had been in their own “state of calamity’” this week over what advice to provide to customers.
“It’s not illegal to travel to an amber light county at the moment as long as you abide by the isolation rules on your return and take into account the cost of the PCR tests required,” she said.
"We urge anyone considering travelling overseas to book with a travel agent who will guide your through the options and ensure that you are well protected against future short or long-term changes.”
Thousands of British football fans are hoping to travel to the Portuguese city of Porto for the all-English Champions League final between Manchester City and Chelsea on May 29.
Uefa previously confirmed 6,000 tickets would be made available to each of the finalists, with the final capacity limit at the Estadio do Dragao still to be fixed.
Portugal is among 12 countries and territories on the quarantine-free green travel list.
Many popular destinations, such as Spain, France, Italy and Greece, are on the amber list, meaning returning travellers will be required to self-isolate at home for ten days.
Non-essential travel from Northern Ireland to the Common Travel Area will be allowed from the same date.