European Parliament leaders have condemned US President Donald Trump's potential choice as envoy to the EU.
Businessman Ted Malloch is believed to be the favourite for the role.
In a letter, the leaders of the main parliamentary groups say he supports the dissolution of the EU and that his views reveal "outrageous malevolence".
One of the leaders separately said that Mr Malloch, who has previously compared the bloc to the former Soviet Union, should be declared "persona non grata".
In an interview with the BBC last month, Mr Malloch said: "I had in a previous career a diplomatic post where I helped bring down the Soviet Union. So maybe there's another union that needs a little taming."
He also said Mr Trump was not a fan of the EU, because it is "supranational" and "unelected".
He has also been a vocal critic of the common currency and European Commission President Jean-Claude Juncker.
A letter, co-signed by the centre-right EPP and the liberal Alde group, said his appointment as US ambassador to could seriously undermine transatlantic relations.
"[Mr Malloch's] statements reveal outrageous malevolence regarding the values that define this European Union," it said.
"We are strongly convinced that persons seeing as their mission to disrupt or dissolve the European Union, should not be accredited as official representatives to the EU," it added.
Mr Malloch has worked at the United Nations in Geneva and been on the executive board of the World Economic Forum, a group of global political and business leaders.
He is currently a professor at Henley Business School at the UK's University of Reading.
Socialists and Democrats leader Gianni Pittella wrote a separate letter, voicing concern about his possible appointment, saying that it could "potentially contribute to the spread of populism and euroscepticism across Europe".
"Malloch should not be accepted as an official representative to the EU and should be declared 'persona non grata'," wrote Pittella.
EU Council President Donald Tusk has said he believes Mr Trump's presidency could be a threat to Europe.
The EPP is the biggest group in parliament, followed by the Socialists and Democrats group. The Alde liberals are the fourth-largest group.
The European Parliament is the only directly elected EU body. It represents the people of the EU in contrast to the Council of Ministers, which represents the governments of member countries.