Image caption Bob Shipp said BP wanted to hire his entire marine science department

The head of the American Association of University Professors has accused BP of trying to "buy" the best scientists and academics to help it contest litigation after the Gulf of Mexico oil spill.

"This is really one huge corporation trying to buy faculty silence in a comprehensive way," said Cary Nelson.

BP faces more than 300 lawsuits so far.

In a statement, BP says it has hired more than a dozen national and local scientists "with expertise in the resources of the Gulf of Mexico".

The BBC has obtained a copy of a contract offered to scientists by BP. It says that scientists cannot publish the research they do for BP or speak about the data for at least three years, or until the government gives the final approval to the company's restoration plan for the whole of the Gulf.

It also states scientists may perform research for other agencies as long as it does not conflict with the work they are doing for BP.

And it adds that scientists must take instructions from lawyers offering the contracts and other in-house counsel at BP.

Bob Shipp, the head of marine sciences at the University of South Alabama, was one of the scientists approached by BP's lawyers.

They didn't just want him, they wanted his whole department.

"They contacted me and said we would like to have your department interact to develop the best restoration plan possible after this oil spill," he said.

Media playback is unsupported on your device Media caption Russ Lea from the University of South Alabama: Some clauses in the contract "were very disturbing".

"We laid the ground rules - that any research we did, we would have to take total control of the data, transparency and the freedom to make those data available to other scientists and subject to peer review. They left and we never heard back from them."

What Mr Nelson is concerned about is BP's control over scientific research.

"Our ability to evaluate the disaster and write public policy and make decisions about it as a country can be impacted by the silence of the research scientists who are looking at conditions," he said.

"It's hugely destructive. I mean at some level, this is really BP versus the people of the United States."

In its statement, BP says it "does not place restrictions on academics speaking about scientific data".

'Powerful economic interests'

But New Orleans environmental lawyer Joel Waltzer looked over the contract and said BP's statement did not match up.

Good scientists, they're going to be giving their opinions based on the facts and they are not going to bias their opinions Professor Irv Mendelssohn, Louisiana State University

"They're the ones who control the process. They're depriving the public of the data and the transparency that we all deserve."

But some scientists who have been approached by lawyers acting on behalf of BP are willing to sign up.

Irv Mendelssohn is a professor in the Department of Oceanography and Coastal Sciences at Louisiana State University.

"What I'm doing wouldn't be any different than if I was consulting with one of the natural resource trustees. I am giving my objective opinion about recovery."

Some scientists approached by BP lawyers have been offered as much as $250 an hour.

Prof Mendelssohn says he would negotiate his normal consulting fee, which is between $150 and $300 an hour. But he says that is not why he is doing it.

"Good scientists, they're going to be giving their opinions based on the facts and they are not going to bias their opinions. What's most important is credibility."

But Cary Nelson is concerned about the relationship between corporations and academia.

"There is a problem for a faculty member who becomes closely associated with a corporation with such powerful financial interests.

"My advice would be: think twice before you sign a contract with a corporation that has such powerful economic interests at stake."