Cannes is smacking its lips in anticipation of filmmaker and provocateur Michael Moore's latest jeremiad against the US administration, which receives its premiere at the film festival today. Sicko, a documentary tackling the state of American healthcare, focuses on the pharmaceutical giants, and particularly on health insurers.

The film has already caused Moore - who won the Palme d'Or at Cannes in 2004 with Fahrenheit 911 - to clash with the American authorities. Now, according to movie mogul Harvey Weinstein, whose Weinstein Company is behind the film, the US government is attempting to impound the negative.

According to Weinstein, the US Treasury's moves meant "we had to fly the movie to another country"- he would not say to where. "Let the secret service find that out - though this is the same country that thought there were weapons of mass destruction, so they'll never find it." He added that he feared that if the film were impounded, there might be attempts to cut some footage, in particular the last 20 minutes, which related to a trip to Cuba. This, said Weinstein, "would not be good."

In March, Moore travelled to the Caribbean island with a group of emergency workers from New York's Ground Zero to see whether they would receive better care under the Castro regime than they had under George Bush. He had applied for permission to travel in October 2006 and received no reply.

In a letter dated May 2, the treasury department notified Moore that it was investigating him for unlicensed travel to Cuba, or, as the missive put it, engaging in "travel-related transactions involving Cuba."

Now team Moore is hitting back. Weinstein has hired an attorney, David Boies, who has lodged a request under the US freedom of information act to find out what motivated the treasury to begin its investigation. "They have to tell us why they did it and what they did," said Weinstein. "And they are not too happy about it."

Weinstein believes the investigation has a political agenda. "We want to find out who motivated this. We suspect there may be interference from another office," he said. "Otherwise, I don't understand why this would have come about."

Weinstein named no suspects in this putative political interference, but referred to outspoken critics of Moore on the Republican right - who tend to accuse him of peddling propaganda rather than of undertaking serious journalism - including presidential hopeful Fred Thompson.

"Senator Thompson has come out with a tirade against Michael. Michael said he'd debate him, but Thompson turned him down," said Weinstein.

He also said that insurers and pharmaceutical companies had "already sent out letters advising employees how to react when the film comes out".

Weinstein appeared to be enjoying the brouhaha that the film is stirring up before it has even screened. "I've already told the Treasury that they are saving me money on advertising."

In Cannes, the Weinstein Company's offices are decorated with a mural of the rotund Moore sitting in a hospital waiting area flanked by a pair of skeletons, and Sicko sticking plasters are being given away as promotional gifts.

Moore's underlying thesis in Sicko relates to the structure of American society. "Others see themselves as a collective that sinks or swims together," he told Variety.

"It's important to have a safety net and free universal health care. In America, unfortunately, we're more focused on what's in it for me. It's every man for himself. If you're sick and have lost a job, it's not my problem. Don't bother me."

The insurance companies are a negative force, he believes. "They get in the way of taking care of those who are ill. They make it worse. We don't need them," he said.

The health secretary, Patricia Hewitt, may be surprised by Moore's ringing - if strictly speaking, factually inaccurate - endorsement for the NHS. "The poorest Brit is healthier and lives longer than the wealthiest American," he said.

Of his journalistic style, he said: "It's the op-ed page. You don't say that's not journalism. I present my opinion, my take on things, based on indisputable facts. They could be wrong. I think they're right." Moore's biggest hit to date has been Fahrenheit 911, which took $222m (£112m) worldwide. He made Bowling For Columbine, his acclaimed film about US gun culture, in 2002. The rightwing backlash has spawned a number of documentaries questioning his methods, including Rick Caine and Debbie Melnyk's Manufacturing Dissent. Moore has hired Al Gore's former press secretary, Chris Lehane, to help him to deal with "the forces I'm up against".

· This article was amended on Wednesday May 23 2007. Former Senator Fred (not Bob) Thompson has criticised Michael Moore. This has been corrected.