Theresa May will have formal meetings with the leaders of all the G7 countries at the group's summit in Canada - except US President Donald Trump.
The US leader is at odds with the other six countries in the group - the UK, Canada, Germany, France, Italy and Japan - over his protectionist tariffs on steel and aluminium imports.
Mr Trump is not expected to spend as long at the summit in Charlevoix, Quebec as other leaders, leaving little time for formal bilateral meetings in front of the world's press.
But Mrs May is expected to have informal talks with the US president and will seek to use the summit to avert a damaging trade war, insisting she wants a "proportionate" response from the European Union to the US tariffs.
The Prime Minister told reporters on the plane to the summit in Quebec: "I made my views clear on the steel and aluminium tariffs that President Trump has announced, I have done that directly to him.
"My most recent conversation with him was on Monday this week.
"We disagree with these, we think they're unjustified.
"Obviously the European Union will be responding.
"We want to ensure, and we're working with others in the European Union to ensure, that this response is proportionate, that it is within the WTO rules.
"As the UK, we want to be a great champion of free trade around the world and that's what we will continue to be.
"I will continue to put the argument for the importance of those trade relationships around the world and I'll be doing that here at the G7 as I have done elsewhere and will continue to do elsewhere."
The Prime Minister will join leaders from Canada, Germany, France, Italy, Japan in voicing concerns with the US president over the imposition of tariffs on steel and aluminium.
The meeting of G7 leaders follows a stormy gathering of the countries' finance ministers earlier this month where the US was left isolated because of the rift caused by the refusal to exempt allies from the 25% tariff on steel imports and the 10% duty on aluminium.