Sir Geoffrey, who lived in the States for 10 years, said Trump had appealed to large numbers of Americans who felt "dispossessed", Sir Geoffrey said, and ran a campaign that was based on fear.
"It was a very dispiriting campaign. I cannot recall any US election in my lifetime that was anything like this one. It was unbelievably negative."
Sir Geoffrey said divisions in the US would not heal easily, and the world had much to fear from a Trump presidency.
"Trump has said he doesn't like the Nato alliance. That will cause many people in Europe to feel deeply insecure, especially with a resurgent Russia.
"The European peace, if you like, since the Second World War was really the result of the United States' Marshall Plan, that put Europe back together again. This is rather serious from that point of view."
Also greatly alarming were Trump's remarks about his country's nuclear arsenal.
"The prospect of using nuclear weapons is not something that anyone wants to contemplate or should contemplate. But he has said things about nuclear weapons - he's said, 'If we've got them, why don't we use them?'"
Sir Geoffrey said Trump's rise had changed US politics forever.
"This man took the primaries by completely attacking the Republican parties...he used the techniques of what I would call reality television...[and] destroyed his opponents.
"The professional politicians and their pollsters have no idea really what happened here...what you are seeing here is a complete change in the political system, and the Western world seems to have less trust in its institutions of representative democracy than it used to. And that's a serious worry."