George Osborne criticised Donald Trump's controversial travel ban as he took up a post at an institution set up by one of the United States president's most prominent Republican critics.
The former chancellor said British citizens will suffer "if the Statue of Liberty turns its back on the world" and "if Britain retreats behind its island shores" as he took up a role at the McCain Institute, set up by US senator John McCain.
It comes amid widespread outrage at Mr Trump's temporary immigration ban on nationals from seven mainly Muslim countries.
Earlier this month, Mr Osborne took up a part-time role as a senior adviser to asset management firm BlackRock and has already earned more than £540,000 for speeches he has given since being sacked from the Cabinet by Theresa May in July.
He is still earning a £74,962 salary as MP for Tatton and has used his position on the backbenches to warn the Prime Minister against pursuing a "hard Brexit" outside the European single market.
In his first article as the McCain Institute's inaugural Kissinger Fellow, Mr Osborne said he recognised the "alluring" temptation for governments to become more inward-looking in the face of financial crisis, globalisation and rapid technological and cultural change.
But attacking Mr Trump, he went on: "John McCain and Henry Kissinger remind us that the world's problems will quickly become our own problems if we leave it to others alone to sort them.
"Erecting trade barriers with our neighbours, making an enemy of our open societies, demonising those seeking a better life, turning away refugees, unravelling the institutions that sustain the West, are not the answer.
"If the Statue of Liberty turns its back on the world, if Britain retreats behind its island shores, then it is not just others who depend on us who will pay a price – the heavy cost will fall on our own citizens too."
Commenting on Mr Osborne's appointment, Mr McCain said: "I am pleased to learn the institute has selected George Osborne as the inaugural Kissinger Fellow.
"George has shown strong and thoughtful leadership throughout his career and proved incredibly able as chancellor.
"At a time when the great democracies of the world are facing challenges such as we have not seen for generations, we need strong, values-driven leaders like George Osborne."
It is understood Mr Osborne will be paid a stipend by the McCain Institute as well as receiving money to cover staff support, travel, convening and conferencing costs.