The final place on the 2021 calendar will go to the Portuguese Grand Prix, according to Autosport magazine citing "sources with good knowledge of the situation".
The FIA unveiled a 23-race season last month, but left the venue for the race now scheduled for May 2 as 'to be confirmed' with a possible second race at Bahrain among the options under consideration.
But after weeks of uncertainty over the current coronavirus situation in the country, reports say that a deal has now been struck to bring F1 back to Portimao for a second time.
F1 teams will be officially informed of the decision to add Portugal to the calendar at Thursday's meeting of the F1 Commission, which is also expected to consider proposals for three Saturday sprint races.
The Autódromo Internacional do Algarve made its debut on the calendar last year as a last minute replacement for long haul races during the COVID pandemic.
The undulating circuit proved popular with drivers, teams and fans, with Mercedes drivers Lewis Hamilton and Valtteri Bottas coming out top in qualifying and the race.
Following the decision to delay the Australia Grand Prix until November and the indefinite postponement of the Chinese GP, it means Portugal is set to be the third round of 2021 after initial outings in Bahrain and Imola.
The Portuguese GP will form a back-to-back double-header with Spain the following weekend, followed by a weekend off before the Monaco GP at the end of the month.
However all plans remain subject to developments with the coronavirus situation and adherence to local, regional and national safety measures.
Portugal is currently on the UK government's 'red list' of countries that will require arrivals from there to quarantine in a hotel for ten days. Currently there is no exemption that that requirement for elite sports.