The EU warned US President Donald Trump on Thursday against letting individual states like Britain off global steel and aluminium tariffs, saying any such exemption would be viewed as applying to the whole bloc.
The White House signalled Wednesday it would water down plans for the contentious tariffs by exempting Mexico, Canada "and possibly other countries as well", in the face of jitters on world markets.
But European Commission Vice President Jyrki Katainen said Brussels would not accept a carve-out for Washington's close ally Britain, which has voted to leave the European Union but remains a member until 2019.
"What I have read today is that they are probably considering some exemptions to NAFTA (North American Free Trade Agreement) countries... but also they have mentioned the UK and maybe some other countries," Katainen told reporters.
"So if they try to make (an) exemption for one of our member states it means the EU as a whole."
Finnish former prime minister Katainen said Brussels had contacted US authorities "intensively" in recent days "to try to convince them not to trigger any major damages to the US economy and the world economy".
The European Union sounded a stern warning Wednesday as it readied to retaliate with US-targeted tariffs of its own on everything from steel to peanut butter, orange juice, cranberries, bourbon and denim jeans.
"Trade wars are bad and easy to lose," EU President Donald Tusk said on Wednesday, directly rebuffing Trump's assertion last week that they were "good and easy to win".