According to medical journal Lancet, the lifespan in Portugal will reach 84.5 in 2040.
Spain is forecast to overtake Japan, ranked first in 2016, which drops into second place with an average lifespan of 85.7 in 2040.
The average life expectancy in Portugal between 2015 and 2017 was 80.78 years, which showed an increase over the previous two years, especially for men.
According to figures from the mortality tables published, men currently have a life expectancy of 77.74 years at birth, an additional 1.6 months, and women 83.78 years, an extra month compared to the previous two-year period.
In ten years, the average life expectancy has increased by 2.28 years for the general population, 2.56 years for men and 1.78 years for women.
In the case of men, mortality below the age of 60 fell, while in women it decreased from aged 60 or more.
The gender gap in average life expectancy decreased from 6.45 years, ten years ago, to 5.67 years in the period between 2015 and 2017.
For those over 65 years of age, the life expectancy of the population was 19.45 years.
Once again, in women this indicator is higher: 20.81 years compared to 17.55 for men.
Meanwhile, according to the study released this week, lifespan gains around the world are likely to be checked by factors such as high blood pressure, diabetes, obesity, tobacco and alcohol use, and air pollution, according to the study.
African countries continue to have the worst rates of premature death.
Citizens of the Central African Republic, Lesotho, Somalia and Zimbabwe are still expected to die before the age of 65 by 2040.
Notably the US makes the biggest drop down the league table, falling from 43rd place in 2016 to 64th in 22 years’ time.
Average life expectancy in the US, one of the world’s richest nations, is forecast to increase by only 1.1 years to 79.8.
Lead author Dr Kyle Foreman, from the Institute for Health Metrics and Evaluation (IHME) at the University of Washington, US, said: “The future of the world’s health is not pre-ordained, and there is a wide range of plausible trajectories. But whether we see significant progress or stagnation depends on how well or poorly health systems address key health drivers.”
To carry out the study, researchers looked at the impact of 79 independent drivers of health including smoking, high body mass index (BMI), and lack of clean water and sanitation on causes of death around the world.
In 2016, the last time the study was carried out, four of the top 10 causes of premature death were non-communicable diseases (NCDs) and injuries.
The new forecast shows that by 2040 this figure increases to eight. Heart disease, stroke, chronic lung disease, chronic kidney disease, Alzheimer’s disease, diabetes, lung cancer and road accident injuries are all in the top 10 causes of premature death in 2040.