Ratings agency Standard & Poor's downgraded Britain's economic prospects as it estimated the country's potential exit from the 28-member bloc.
The ruling conservative party has long been skeptical about the euro, and as the referendum approaches, its ruling is at the risk of being weakened.
Moris Kraemer, chief sovereign ratings officer at S&P, said in an interview with CNBC:
"What we believe we are observing in the UK is similar to what we saw in the US in 2011 when we downgraded its outlook. Policymaking may be at risk at being subordinated to party politics – being driven not by what is best for the country, but for a particular constituency."
According to the agency, the country is at around a third risk of downgrade within the next two years.
The agency expressed confidence about British economic recovery in 2014, as it restored the U.K.'s credit rating to AAA: Stable.
Frictions from the side of the Scottish National Party are another evidence the horizon is not clear.
The S&P also noted that while the U.K. has been successful at attracting foreign investment being a part of the EU, the situation may change if it exits the union, as "there must be some consequences in terms of decision making by corporate captains."
Mr Kraemer also said that alternatives for the EU can be hardly found for the U.K.
The U.K. could move to membership of the
European Economic Area, like Norway, and use the added freedom to
negotiate its own trade deals with major markets like the U.S. and
China, for instance.
But "these countries have to implement policies without having a seat at the board to decide them," Kraemer warned.
"Britain wants to be careful in losing influence in Europe over the direction of policy."
