The senior Whitehall mandarin told an Institute for Government event: “One fact that will not change is that we are a medium sized country and most of what we want to do on the international stage we can only achieve with partners.”
And in what appeared to be a coded attack on Brexit, he also bemoaned the “hard work” that Brexit will make for the Foreign Office.
Sir Simon said: “Outside the EU, outside our regional club it is going to be more difficult to do what we want so we are going to have to work harder.”
“For the last 45 years we have been a member of a big pretty coherent regional club and been one of the three biggest players in that club and that starts on the 29 March next year so I think that will certainly make everything where the EU has led particularly round trade more difficult for the UK.”
But Mr Seely, a member of the foreign affairs committee, said: “I just get so wound up when I hear this medium sized power stuff.”
While he conceded that the world’s two superpowers are the USA and China he pointed out that Britain ranks in the next tier of “great nations” above even Russia whose power is only measured in “destructive” military might.
Mr Seely said: “We are in many ways the second most powerful nation on earth in terms of our soft power. So can we please nail this we are a little country routine. I don’t think it is going to help us post Brexit.”
The Isle of Wight MP was commenting ahead of a publication of a paper he has produced for the Henry Jackson Society on the definition of Russian warfare.
Defending himself, Sir Simon was forced to admit that Britain is still a nation of consequence.
He said: “I don’t think medium sized is small. We are in the league table in the top half dozen for the size of the economy, the size of our armed forces, our aid development assistance is in the top three.
“So we are in weight and consequence but the top two are way ahead so it’s that relative thing we need to be honest about.”
Sir Simon also signalled that Government departments are already beginning to argue over what will happen to the Brexit Department’s money and resources after the UK leaves the EU saying that the Foreign Office expects to get “a large chunk” of it.
The creation of the Brexit Department to run the negotiations was known to have been opposed in some parts of the Foreign Office which saw it as removing part of its responsibilities.
Sir Simon is paid more than £180,000 to run the Diplomatic Service after a Foreign Office career which began in 1982 and has included him serving as Jack Straw’s principal private secretary, the UK’s ambassador in Israel and later Germany, and the Director for Iraq during the war.
The confrontation between Sir Simon and Mr Seely has come against a background of concern that senior figures in Whitehall are opposed to Brexit and do not want it to succeed.
Previously the ire of Brexit supporting MPs has been directed at the Treasury which played a major role in producing inaccurate doomsday scenario forecasts of how Britain’s economy would plummet as the result of a Leave vote which never transpired.
There were concerns as well over a partial leak of a document from the Treasury which appeared to suggest that the least worst option for Britain was to stay under Brussels rule in the single market and a customs union.