Former British Prime Minister John Major | Pool photo by Brian Lawless/Getty Images John Major: Brexit makes UK reliant on Donald Trump ‘A little more charm, and a lot less cheap rhetoric’ would help the UK in trade talks, says former prime minister.

A United Kingdom outside the EU will become more reliant on the United States and upon a president "less predictable, less reliable and less attuned to our free market and socially liberal instincts than any of his predecessors,” former British prime minister John Major said Monday.

“Once we are out of the EU, our relationship with the United States will change. She needs a close ally inside the EU: Once outside, that can no longer be us,” Major said in a speech at Chatham House in London.

Major, who has become a leading voice in the campaign against the Brexit vote and late last year claimed there was a "perfectly credible" case for a second referendum, said the vote had done "great harm" to the EU and warned of a domino effect across Europe. He said Brexit had "energized the anti-EU, anti-immigrant nationalists" ahead of elections in the Netherlands, France and Germany this year.

“None of these populist groups is sympathetic to the broadly tolerant and liberal instincts of the British. Nonetheless, their pitch is straightforward. If Britain — sober, stable, moderate, reliable Britain, with its ancient parliament and anti-revolutionary history — can break free of a repressive bureaucracy in Brussels ... then 'so can anyone.' It is a potent appeal.”

With the current prime minister, Theresa May, pledging to trigger Article 50 by the end of March, Major said the atmosphere between the two sides "is already sour," adding that "a little more charm, and a lot less cheap rhetoric, would do much to protect the U.K.’s interests.”

"There is a real risk the outcome [of the trade talks] will fall well below the hopes and expectations that have been raised: I see little chance we will be able to match the advantages of the single market.”

"Negotiations are all about give and take. We know what the Brexiteers wish to take: Yet we hear nothing about what our country may have to give in return."

He also spoke about Remain voters — many of whom "have written to me in dismay, even despair” — saying "it's not 'arrogant' or 'brazen' or 'elitist' or remotely 'delusional' to express concern about our future after Brexit.

“I have watched with growing concern as the British people have been led to expect a future that seems to be unreal and over-optimistic. Obstacles are brushed aside as of no consequence, whilst opportunities are inflated beyond any reasonable expectation of delivery.”