EU competition commissioner Mario Monti has condemned as "entirely out of place" comments over the tough line he has said to have taken over the $42bn deal, between two of America's biggest companies.
"This is a matter of law and economics, not politics," he said.
"We have been reviewing mergers and acquisition for over 120 years, and each time the commission has applied the same principles and the same market dominance test."
"I deplore attempts to misinform the public and to trigger political intervention."
US concern
Mr Monti's outburst came the same day that Robert Zoellick, US trade representative, added to US pressure on the EC to approve the merger, which has already been passed by US and Canadian anti-trust chiefs.
At the Paris Air Show, Harry Stonecipher, vice chairman of Boeing, said he was "hopeful that [the deal] will still work out".
And last week Mr Bush said he was "concerned that the Europeans have rejected" the merger.
The US comments followed a statement from General Electric on Friday claiming that the EC had made "extraordinary demands" in negotiations over the merger, raising fears that the commission would block it.
Widespread concerns
But Mr Monti stressed that the EU, which has a deadline of 12 July to comment on the merger, has not yet made a decision.
And he said that the dispute was not simply a US versus EU matter.
"The merger has raised strong concerns among suppliers and customers, particularly airlines, on both sides of the Atlantic," he said.
"This is not a transatlantic dispute."
American line
Mr Bush's office on Monday appeared to be attempting to calm US/EC tension, denying that the president had sought to interfere with Mr Monti's anti-trust procedures.
"[Mr Bush] reiterated the American position, which [was that] the American government already cleared the merger so, of course, the president said that," White House spokesman Ari Fleischer said.
Mr Bush had merely "reiterated the American position", Mr Fleischer said.