Rating agency Fitch downgraded U.K.'s sovereign rating to 'AA-' from 'AA' on Friday, blaming a significant weakening of the country's public finances due to the coronavirus outbreak and a fiscal loosening stance.

The rating change comes as Prime Minister Boris Johnson tested positive for the coronavirus, which has infected more than 14,500 and killed 759 as of Thursday afternoon in the U.K., up by nearly a third in 24 hours.

The agency maintained the country's outlook at 'negative.'