Britain's outgoing ambassador to the European Union has said Prime Minister Theresa May's negotiating objectives for Brexit were unknown to her government's representatives in Brussels.

In a letter to staff announcing his resignation less than three months before London triggers formal exit talks, envoy Ivan Rogers made a scarcely-veiled attack on the government's Brexit strategy, which he implied had not been thoroughly prepared. He referred to "the uncertainty that I know, from our many discussions in the autumn, you are all feeling about the role of UKRep in the coming months and years of negotiations over Brexit".

In a revelation bound to be embarrassing to the government, he said: ""We do not yet know what the government will set as negotiating objectives for the UK's relationship with the EU after exit."

He also warned that EU negotiators would be better resourced than Britain's, saying: "Serious multilateral negotiating experience is in short supply in Whitehall, and that is not the case in the [European] Commission or in the Council."

In an implicit criticism of his political masters, Rogers told his team: "I hope you will continue to challenge ill-founded arguments and muddled thinking and that you will never be afraid to speak the truth to those in power."

Evidently attacking an ideologically-based approach to Brexit, he said: "Contrary to the beliefs of some, free trade does not just happen when it is not thwarted by authorities: increasing market access to other markets and consumer choice in our own, depends on the deals, multilateral, plurilateral and bilateral that we strike, and the terms that we agree. I shall advise my successor to continue to make these points."

Rogers has previously said a trade deal with the EU might take 10 years to strike, angering Brexit supporters who believe it will be a straightforward process. 

Additional reporting by Reuters.