Guy Warrington was quoted on Friday saying that if the economy is to improve, the government must encourage foreign investors, citing issues like a strong legal system and the signing and respect of international treaties that seek to protect the interest of investors.
“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 exist 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.
The UK, a former colonial power in Sierra Leone, is leading aid provider for the West African country but the two have intermittently had tough relations, most recently before and during the 2018 general elections.
The former administration of Ernest Bai Koroma accused the UK of involvement in an international plot to effect a regime change policy.
They claimed Mr Warrington was used to implement the policy.
The former ruling All Peoples Congress, which is now the main opposition, even called for his deportation.