Fitch Ratings has downgraded the UK from AAA to AA+, but said that the outlook is stable.
The rating actions follow the conclusion of the review of the UK's sovereign ratings initiated on 22 March.
Fitch has explained that the reason for the downgrade is that despite the UK's "strong fiscal financing flexibility", its ability to absorb further adverse economic shocks is "no longer consistent with an 'AAA' rating".
More top news
Choir wins public vote, while judges pick dancers ahead of 12-year-old singer-songwriter Henry Gallagher.
The hunt for murder suspect Jed Allen has ended after police found his body two days after discovering his mother, stepdad and sister dead.
A woman who had been searching for her mother for more than 10 years has been reunited with her after posting an appeal on social media.