Ratings agency Standard & Poor's said the referendum could lead to "a deterioration of the UK's economic performance, including its large financial services sector". S&P had been the only major agency to maintain a AAA rating for the UK, which has now been given a AA rating. Moody's downgraded the UK's credit rating outlook to negative on Friday 24 June.
A rating downgrade can affect how much it costs governments to borrow money in the international financial markets. In theory, a high credit rating means a lower interest rate. S&P said that the leave result would "weaken the predictability, stability, and effectiveness of policymaking in the UK", according to an AP report. The UK is now considered less credit worthy than the US and EU by S&P.
Economic growth in the UK could be adversely affected by the vote, said the ratings agency. "We take the view that the deep divisions both within the ruling Conservative Party and society as a whole over the european question may not heal quickly and may hamper government stability and complicate policymaking on economic and other matters," it said.
The ratings agency also warned of a "constitutional crisis" if there would be a second referendum on Scottish independence from Britain. Reflecting the mood of business sentiment, The Institute of Directors said that 20% of its members plan to move out of Britain some of their operations due to the Brexit vote. The IOD also said that three out of four of its members felt that the UK leaving the EU was bad for business.
Sterling continued to decline against the dollar, to $1.31, a level not seen since 1985, the largest two-day decline in history. Chancellor George Osborne spoke before the markets opened on Monday, saying: "Markets move up, markets move down. We don't yet know where they will find their level."
He added: "What we need is a bit of calm now, there's no reason for any of us to panic."