In a damning indictment of the government's negotiation strategy, Moody's said its failure "to manage change in a predictable and confidence-building manner is evident with respect to the UK's approach to Brexit, in its inability to achieve an outcome which meaningfully replicates the benefits of EU membership and in its approach to implementing the agreement reached with the EU to date".

"Even if there is a trade deal between the UK and EU by the end of 2020, it will likely be narrow in scope, and therefore the UK's exit from the EU will, in Moody's view, continue to put downward pressure on private investment and economic growth," the agency added.