By Maria Armental 

Standard & Poor's Global Ratings stripped the U.K. of its pristine triple-A credit rating on Monday, following through on its warning that last week's vote to leave the European Union threatens the country's constitutional and economic integrity.

The firm, which cut the country's ratings by two notches to double-A, also said the vote for "remain" in Scotland and Northern Ireland creates wider constitutional issues for the country as a whole. S&P's ratings outlook is negative, which signals a potential for additional downgrades.

The U.K.'s surprise decision to leave the EU has rattled stock, currency and debt markets around the world, while also leading to a political upheaval in the U.K.

S&P was the last of the so-called Big Three rating firms to withdraw the U.K.'s triple-A rating. Its downgrade comes after rival firm Moody's Investors Service changed its outlook on the U.K.'s ratings to negative from stable on Friday, while affirming them at Aa1, the second-highest level. Fitch Ratings has the U.K. at an equivalent double-A-plus.

S&P had warned Friday that it was reviewing the ratings and said it believed the vote to leave the EU would deter investment in the economy, decrease demand for sterling reserves and put the country's financial services sector at a disadvantage with other financial centers.

A double-A rating places the U.K. in the same basket as Belgium and France, but behind the likes of Germany, the Netherlands and Australia, all of which retain the coveted triple-A grade from S&P. The credit rater has the U.S. at double-A-plus, the second-highest mark.

S&P, for its part, said it believes EU membership has helped the U.K. economy attract more low-cost capital and skilled labor and bolstered London's position as a global financial center, boosting in turn the sterling as a reserve currency. However, the sterling, whose value has collapsed in the aftermath of the 'Brexit' vote, could lose its reserve-currency status if central banks move away from the currency. The sterling accounted for about 4.9% of central banks' foreign currency reserve holdings as of the fourth quarter, according to International Monetary Fund data.

The U.K.'s economy, S&P noted, has expanded faster over the past two years than almost all of the large European economies, including Germany's.

"However, given the uncertainty and fall in investment tied to the 'leave' vote, we are forecasting a significant slowdown in 2016-2019," the rating firm said, projecting gross domestic product expansion to average 1.1% a year, down from its April forecast of 2.1%.

Pressure on the sterling, however, could help narrow the import-export gap by weakening imports as economic growth decelerates, the rating firm said, projecting current account deficit to average 3.4% in 2016 to 2019, compared with its April forecast of 4.5%.

In 2015, current account deficit accounted for 6.7% of GDP.

Write to Maria Armental at maria.armental@wsj.com

(END) Dow Jones Newswires

June 27, 2016 15:49 ET (19:49 GMT)

Copyright (c) 2016 Dow Jones & Company, Inc.