The global credit agency Standard & Poor has downgraded the UK's credit rating from the top rating of AAA to AA, following Britain's historic vote to leave the EU.

The long-term sovereign credit decrease represents a two-notch fall, and follows the UK's credit outlook being adjusted from "stable" to "negative" by Moody's.

It said in a statement: "In our opinion, this outcome is a seminal event, and will lead to a less predictable, stable, and effective policy framework in the UK.

"We have reassessed our view of the U.K.'s institutional assessment and now no longer consider it a strength in our assessment of the rating.”

S&P said the disparity of voting within the UK, with both Scotland and Northern Ireland voting to remain, could cause "wider constitutional issues for the country as a whole."

It continued: "The negative outlook reflects the risk to economic prospects, fiscal and external performance, and the role of sterling as a reserve currency, as well as risks to the constitutional and economic integrity of the U.K. if there is another referendum on Scottish independence."

Brexit reactions – in pictures Show all 10 1 /10 Brexit reactions – in pictures Brexit reactions – in pictures Supporters of the Stronger In campaign look at their phones after hearing results in the EU referendum at London's Royal Festival Hall AP Brexit reactions – in pictures Leave supporters cheer results at a Leave.eu party after polling stations closed in the Referendum on the European Union in London Reuters Brexit reactions – in pictures Mr Cameron announces his resignation to supporters Getty Brexit reactions – in pictures Donald Tusk proposes that the 27 remaining EU member states ‘start a wider reflection on the future of our union’ Getty Brexit reactions – in pictures Ukip leader Nigel Farage greets his supporters on College Green in Westminster, after Britain voted to leave the European Union PA Brexit reactions – in pictures Supporters of the Stronger In Campaign react as referendum results are announced today Getty Brexit reactions – in pictures Boris Johnson leaves his home today to discover a crowd of waiting journalists and police officers Getty Brexit reactions – in pictures Leave EU supporters celebrate as they watch the British EU Referendum results being televised at Millbank Tower in London Rex Brexit reactions – in pictures Supporters of the Stronger In Campaign react as results of the EU referendum are announced at the Royal Festival Hall Reuters Brexit reactions – in pictures Supporters of the Stronger In campaign react after hearing results in the EU referendum at London's Royal Festival Hall PA

S&P's rating is a marker of how the UK is viewed by international investors, but it is not a change that is likely to carry too much sway with markets in the short term.

The value of the pound slipped almost 2 per cent on Monday morning as the EU referendum result reverberated around global markets.

A speech by the Chancellor George Osborne prompted a slight recovery as the stock market opened, with the pound trading 1.7 per cent down at $1.3497.