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

"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 UK if there is another referendum on Scottish independence."

The loss of the last remaining "AAA" rating represents a fresh blow to Britain's economic standing after the referendum.

It comes after Sterling fell to a 31-year low against the dollar and the country's stock markets plunged.

Rival ratings agencies Fitch and Moody's stripped Britain of their AAA ratings long before the referendum campaign began. 

And on Friday it changed the UK's sovereign rating from "stable" to "negative", stating that the unpredictability of UK decision-making factored into its move.

This evening, it further reduced its rating from AA+ to AA.

"Fitch believes that uncertainty following the referendum outcome will induce an abrupt slowdown in short-term GDP growth, as businesses defer investment and consider changes to the legal and regulatory environment," it said in a statement.

"Medium-term growth will also likely be weaker due to less favourable terms for exports to the EU, lower immigration and a reduction in foreign direct investment. An adjustment in the value of sterling and changes in the business environment could also affect growth."

Protecting Britain's credit rating was a top priority of Conservative finance minister George Osborne when he came to power in 2010.

S&P had been the only major ratings agency to maintain a triple-A rating for Britain.