The mutant strain was found in four people who flew into Japan from Brazil. The passengers tested positive for Covid-19 in airport quarantine, the ministry said.

One of them was a man in his 40s, who showed no symptoms on arrival, but was later hospitalized with breathing difficulties. Another was a woman in her 30s who reported having a headache. A male, aged between 10 and 19, had fewer signs of the illness, while a female in the same age group was asymptomatic.

The new variant of the virus was detected by National Institute of Infectious Diseases (NIID), which thoroughly studied the passengers' samples.

The NIID said that the mutant strain had similarities to the variants of the virus that were discovered in Britain and South Africa in recent weeks and quickly spread to other countries. However, it pointed out that it currently has no evidence to suggest that the variant found in Japan is highly contagious.

Experts have been studying the strain to determine whether it could cause more serious symptoms or be resistant to vaccines. Cases of Covid-19 have been on the rise in Japan since the start of the year, leading to the declaration of a month-long state of emergency in Tokyo on Thursday.

The UK strain of the virus has also been found in the country, despite Japan being among the nations to ban travel from Britain.

Since the start of the pandemic, Japan has recorded more than 283,000 infections and over 3,800 deaths related to the coronavirus.