"The European Union has treated the United States very unfairly when it came to trade," Trump told Britain's ITV channel in the interview conducted Thursday and aired yesterday.
"I've had a lot of problems with (the) European Union, and it may morph into something very big from that standpoint -- from a trade standpoint."
Trump delivered the warning during a wide-ranging interview on the sidelines of the World Economic Forum in Davos, where he took his "America First" agenda to the global business elite.
In a speech on Friday he told the forum that his mantra "does not mean America alone" and hinted that the US could rejoin the Trans-Pacific Partnership, a deal he withdrew from a year ago.
But earlier this month the Trump Administration imposed steep tariffs on imported washing machines and solar panels, and his interview comments may cause alarm in European capitals over future trade relations with the US.
Last year it vowed to impose nearly 300 per cent punitive tariffs on airplanes manufactured by Canada's Bombardier.
A bipartisan US trade panel blocked that decision on Friday but the dispute, which has inflamed relations with Ottawa -- and to a lesser degree Britain, where Bombardier has a large workforce -- could be a harbinger for the EU.
"We cannot get our product in. It's very, very tough. And yet, they send their product to us -- no taxes, very little taxes. It's very unfair," Trump added.
"They're not the only one, by the way, and I could name many countries and places that do (the same).
"But the European Union has been very, very unfair to the United States. And I think it will turn out to be very much to their detriment."
In other remarks aired yesterday, Trump appeared to slight British Prime Minister Theresa May's handling of fraught Brexit negotiations, declaring that he would have "negotiated it differently".
"I would have had a different attitude," he said of the talks, which have followed Britain's June 2016 referendum vote to leave the EU, and will continue through to its planned departure in March 2019.
"I think I would have said that the European Union is not cracked up to what it's supposed to be. And I would have taken a tougher stand in getting out," Trump added.
But May will welcome his prediction -- reinforced by US Treasury Secretary Steven Mnuchin in Davos -- that the US could swiftly strike a post-Brexit trade deal with London.
"We are going to make a deal with (the) UK that'll be great," he said, noting constraints imposed by the Brexit process.
"When that restriction is up we're going to be your great trading partner."
The US president also apologised for the first time for retweeting a British far-right group's videos apparently showing Islamist violence.