Brexit will make it “more difficult” for the United Kingdom to exert influence in the world, the Foreign Office’s top diplomat has warned.

Sir Simon McDonald said on Monday EU membership allowed Britain to make its voice heard “more plausibly” across the world.

The senior civil servant also admitted Theresa May and Boris Johnson’s post-Brexit “global Britain” slogan had “shortcomings”.

Speaking at the Institute for Government think-tank in London, Sir Simon said: “This departure will not signal a diminution in British overseas ambition.

“Because we are leaving the EU, I expect more action in other international organisations

“Because one fact that will not change is we are a medium sized country and most of what we want to do on the international stage we can only achieve with partners.

“We are conscious that outside the EU, outside our biggest regional club, it is going to be more difficult to achieve what we want. So we are going to have to work harder.”

Sir Simon added this would be notable when it came to international trade. “We will be making our way as a country of 65 million people rather than a bloc of more than half a billion,” he said.

“It’s not an impossible task,” the veteran diplomat said. “But I absolutely agree we have to make some choices.

“When you are part of a bigger club with bigger resources you can spread yourself more plausibly across the whole world, we will have to prioritise.”

The prime minister has dubbed her post-Brexit strategy for the UK as “global Britain”.

But Sir Simon said there “clearly have been shortcomings with the label” due to how it has been interpreted.

He said it had not been intended to appear as if the UK was “turning our back on Europe” or attempting to create an “Empire 2.0″.

“For us, ‘global Britain’ is about being present, engaged and effective around the whole world.”

Sir Simon also said he was already jockeying to get hold the money currently given to David Davis’ Department for Exiting the EU (DExEU) once it is dissolved after Brexit.

“The Foreign Office will be making a claim on quite a chunk of DExEU’s work,” he said. “I foresee the division of spoils will be between several players.”