Credit ratings agency Moody's cut Britain's outlook from "stable" to "negative" on Saturday following the country's vote to leave the European Union.
The vote and resignation of Prime Minister David Cameron "will herald a prolonged period of uncertainty for the UK, with negative implications for the country's medium-term growth outlook," US-based Moody's said.
The move came after stock markets plummeted around the world on Friday and the pound fell to levels not seen since 1985.
"The economic and fiscal consequences of the referendum result are highly uncertain as they depend crucially on the outcome of future negotiations with the EU as well as with other trading partners," Moody's said.
The agency cited three factors that could lead to a downgrading of Britain's credit rating: If trade negotiations are protracted and there are suggestions access to the EU's single market could be significantly restricted; if there is little progress in reducing the budget deficit; and if the currency comes under stronger pressure amid "substantial and persistent capital outflows."