Standard & Poor's said Friday it switched to negative the outlook on Britain's top AAA credit rating due to the government's decision to hold a referendum on whether to stay in the European Union.

"We believe that the UK government's decision to hold a referendum on EU membership by 2017 indicates that economic policymaking could be at risk of being more exposed to party politics than we had previously anticipated, similar to the situation in the US when we lowered that sovereign rating in 2011," the credit ratings agency said in a statement.