"The UK vote to leave the European Union in the referendum on 23 June will have a negative impact on the UK economy, public finances and political continuity," the release explained.
The rating agency has also downgraded the Bank of England to 'AA' from 'AA+.'
Fitch said the Brexit vote will likely cause an abrupt slowdown in the UK’s short-term GDP growth, while also undermining medium-term perspectives.
"Fitch has revised down its forecast for real GDP growth to 1.6% in 2016 (from 1.9%), 0.9% in 2017 and 0.9% in 2018 (both from 2.0% respectively), leaving the level of real GDP a cumulative 2.3% lower in 2018 than in its prior 'Remain' base case," the release stated.
On Thursday, the United Kingdom held a referendum to determine whether the country should leave the European Union. According to the final results, 51.9 percent of UK voters, or 17.4 million people, decided to support Brexit, while about 16.1 million opposed it.
The International Monetary Fund (IMF) said in a report on Monday that the decision of the United Kingdom to leave the European Union will likely result in disruption of trade and financial flows.