Buckingham Palace said on Wednesday that the 99-year-old Prince Philips has been admitted to the private King Edward IV hospital on Tuesday evening.
Palace has said his illness is not believed to be related to COVID-19.
Palace said the Duke of Edinburgh is expected to remain in a hospital for s few days of observation and rest.
He and her wife Queen Elizabeth received the first dose of vaccine against the coronavirus in early January.
Philips retired from public duties in 2017 and is rarely seen public.
He has been staying at Windsor Castle with the queen during the current coronavirus lockdown.