The US Government sought to influence the European Commission over Microsoft's antitrust case, according to competition commissioner Neelie Kroes.
Kroes said the US embassy in Brussels had asked her to be "nicer" to Microsoft ahead of her decision to fine the software giant €280m in July.
The commissioner criticised the approach. "This is of course an intervention which is not possible," Kroes told Dutch newspaper Financieele Dagblad this week.
When asked if she was annoyed by the Embassy's approach, she said: "In my work, I cannot have a preference. I have, however, a personal opinion, but that is for Saturday night."
Kroes' spokesperson added in an email to ZDNet UK on Tuesday: "We can confirm that she was lobbied and that she did not appreciate it."
Microsoft declined to comment on Kroes's claims, but an insider insisted that the company had not tried to influence discussions between the US Government and the EC.
The US Embassy had not responded to requests for comment at the time of writing.
Microsoft was hit with the fine for failing to comply with the Commission's landmark antitrust ruling of 2004.
The Commission had ordered the software giant to open up its code to help rivals' server software to be fully compatible with Windows.
Microsoft has previously denied failing to comply with the terms of the antitrust ruling. Two months ago, it submitted a lengthy document which it said proved it had opened up its server interoperability protocols, as demanded by the EC.