The Prime Minister said she had used the event as a chance to register the UK’s “deep disappointment” with President Trump’s plans.
Mrs May has acted as something of a mediator in heated talks at the international summit in Quebec, pleading with fellow leaders to step back from the brink of a trade war at a session on Friday night.
But she directly addressed the issue at a press conference held shortly after the close of the two-day summit.
In a thinly veiled swipe at the United States, the PM said “not all countries play by the rules” and insisted “just because a country has issues with its trading partners, it doesn’t mean that you take unilateral against them”.
She said: “Where we disagree with our allies on something it is right that we say so, and air the issue openly and frankly.
“We have done just that at this summit, registering our deep disappointment at the unjustified decision by the US to apply tariffs to EU steel and aluminium imports.
Theresa May went on to warn the loss of trade through tariffs “ultimately makes everyone poorer”.
And she urged against what she described as “tit-for-tat” escalation of a potential trade war between the US and EU, before calling for a continued “constructive dialogue”.
She added: “As a champion of free trade the UK will continue to support these efforts.”
The PM also said she had spoken to Mr Trump “briefly” at the summit about his upcoming visit to the UK.
Asked whether she was concerned the President could pull out of the trip if a trade war with the EU escalates, Mrs May said Mr Trump had said was looking forward to the trip.
This year’s G7 has been dominated by discussions on Donald Trump’s tariffs, which he insists are for national security reasons.
Speaking before departing early from the summit this afternoon, a furious President Trump vowed he would no longer allow the likes of the EU to treat the United States like a “piggy bank”.
The UK and the rest of the European Union will be slapped with a 25 percent duty on steel and 10 percent on aluminium.
The EU has pledged its own set of retaliatory tariffs, which would come into force next month and target American exports such as bourbon whiskey and jeans.
Ahead of the G7 summit, which began on Thursday, Donald Trump also caused fresh controversy by suggesting Russia be re-admitted to the G7.
Vladimir Putin was expelled from the meeting in 2014 following his illegal annexation of Crimea, and Mrs May has opposed
A senior UK Government source yesterday reminded Mr Trump of the reasons for Russia’s absence from the G7.
The source said: “We should remind ourselves why the G8 became the G7 – it was after Russia illegally annexed Crimea.
“Since then we have seen malign activity from Russia in a whole variety of ways, including on the streets of Salisbury in the UK.”