“I think what investors need for any country they invest in is predictability and I think that predictability needs some improvement,” he said.

He added: “When someone makes a 20-year investment they should be confident that the condition they agreed will be respected by whatever government that exists in that period.”

Mr. Warrington was speaking as he rounds up his three-year diplomatic tour of Sierra Leone. He is scheduled to return to the UK at the end of August. 

Already the UK government has announced the appointment of a replacement, Simon Mustard, who is due to take office in September.