Standard & Poor's has stripped the United Kingdom of its top credit grade in the wake of the vote to leave the European Union.

The rating agency downgraded the country's sovereign rating by two notches, from AAA to AA, saying the vote is a "seminal event" that "will lead to a less predictable, stable and effective policy framework in the U.K."

It is also keeping a negative outlook on the rating, which means it could downgrade the country further.

It added in a report published Monday that the outlook reflects the risk to the economy and public finances, as well as the pound's role as an international reserve currency.

It also cited "risks to the constitutional and economic integrity of the U.K." as Scotland's strong vote to remain in the EU could raise the prospect of another referendum on Scottish independence.