Assessments and forecasts of doses of external exposure to radionuclides deposited in the terrestrial natural and anthropogenic environments are often challenged by diverse environmental and social factors, which are unknown at the time of a nuclear emergency or shortly after but may substantially increase uncertainty of dosimetric estimates. The decade passed since the accident at the Fukushima Daiichi Nuclear Power Station (FDNPS) in 2011 demonstrated that the observed dynamics of ambient dose rates and the long-term forecasts of external exposures in Japan vary from those earlier observed after nuclear weapon tests, radiation accidents and other events of radiological emergency. Those earlier events provided the foundation for the model of external exposure in the UNSCEAR 2013 Report ‘Levels and effects of radiation exposure due to the nuclear accident after the 2011 great east-Japan earthquake and tsunami’. For the new UNSCEAR 2020/2021 Report vol. II ‘Levels and effects of radiation exposure due to the accident at the Fukushima Daiichi Nuclear Power Station: implications of information published since the UNSCEAR 2013 Report’, this model has been critically reviewed and further developed, using results of extensive systematic radiation monitoring in various environments and population-based studies with personal dosimeters conducted by experts in Japan. The new monitoring data created a supporting framework for validation of the improved models for estimation of population external doses from radionuclides deposited on the ground. The revised model follows the generic framework compatible to existing approaches, while considering country-specific features important for the public dose assessment. The model has been applied for forecasting external doses and their uncertainties due to unknown future trends of dose rate dynamics or population behaviour. The new model can be effectively applied for assessment of cumulative external doses and related uncertainties for various exposure scenarios, e.g. for evacuated members of the public returning to their homes, members of population born after the accident, environmental changes occurring as a result of countermeasures or remedial actions.