Boris Johnson appeared directly at odds with US Republican presidential nominee Donald Trump on a key Nato issue during his first visit to Washington as Foreign Secretary.
Speaking as Mr Trump was making his keynote address to the Republican Party convention, Mr Johnson voiced support for the military alliance's principle of mutual defence after the US presidential hopeful raised questions about whether America would come to the aid of the Baltic states if Russia attacked them.
Mr Trump said the US would defend them "if they fulfil their obligations to us".
Mr Johnson, who is in the US capital to discuss combating Islamic State, stressed that automatic mutual defence was the bedrock of Nato and had been responsible for keeping the peace in Europe for decades.
"Let me just reassure you, and reassure everyone on that point - I think that Article Five in the Nato Treaty of 1948, the doctrine of mutual defence, is incredibly important.
"It's something that I've repeated several times already just in the last week to various other countries, and my counterparts in various other countries around Europe, in the Baltic countries and elsewhere.
"It's something that the British Government believes in absolutely, fervently, and that we stand behind full square.
"Fundamentally, it is the Nato Treaty, that doctrine of mutual defence, that has guaranteed the peace in Europe for decades, and will do, I think, for decades to come," Mr Johnson said.
Asked if Mr Trump had made a gaffe with his comments, Mr Johnson said: "It's not for me to get involved in the politics of this election campaign going on in America."
Mr Johnson insisted none of his counterparts on the international stage had mentioned his past insults against world leaders since he became Foreign Secretary.
"Absolutely nobody here today, or indeed in the last few days, has brought up anything to do with that kind of stuff," he said.
Mr Johnson said that although the "foreign policy establishment" had opposed Brexit, the world was now coming to realise that it presented opportunities.
The Foreign Secretary said US secretary of state John Kerry had been encouraging about Britain's new role.
"What John Kerry said to me, a week ago, when I got this job, is what he and the Americans were wanting to see was more UK, not less UK. In other words, he sees this, and the United States see this, as an opportunity to have more of a role on the world stage, and I really agree with him.
"It does not mean that we are turning our backs on the world. On the contrary, we have massive influence, we have massive interest around the world. Getting out of the EU treaties does not mean that Britain leaves Europe, broadly conceived," Mr Johnson said.
He said the UK was committed to fighting the Islamic State threat and the Foreign Office had set up a special unit to combat the group's communications networks.