Lawyers for patients who say they have been harmed by GlaxoSmithKline Plc's Avandia say critical data is still under wraps even as a U.S. advisory panel prepares to weigh the fate of the controversial diabetes drug.

The lawyers, who represent thousands of patients or family members suing the British drugmaker in multi-district federal litigation, say they have uncovered critical documents.

They are pushing the court to allow their release to both the U.S. Food and Drug Administration and the Senate Finance Committee, which has investigated Glaxo's handling of Avandia.

An FDA advisory panel is already wading through hundreds of pages of conflicting data and studies, and is due to meet for two days starting Tuesday on whether to recommend that Avandia be pulled off the market due to heart risks.

Avandia has been under fire since 2007 over concerns that it raises the risk of heart attack, stroke and death more than Takeda Pharmaceutical Co's rival diabetes pill Actos.

It is unclear what the court-sealed documents contain, but it raises questions about whether the FDA will be acting on Avandia without the benefit of all available information.

We think there are very important documents involved, said Joseph Zonies, one of the lead plaintiffs lawyers in the federal case, which is pending in a U.S. district court.

But unless the court lifts current confidentiality protections, the soonest the documents could come to light would be September 5, at a scientific court briefing, he added.

A court-appointed master is considering the request to lift the documents' confidentiality, but it is unclear when that decision will be made and how it would impact the drug's fate.

Glaxo has defended Avandia, saying the overall data shows Avandia does not increase heart attacks or related problems.

Still, it faces numerous lawsuits from patients and family members who allege that use of Avandia led to injury or death. Zonies estimated 3,000 cases have been filed in federal litigation while another 4,000 to 6,000 could be filed later.

FDA officials have said they've reviewed all the data possible in assessing Avandia's risks. On Friday, they released more than 700 pages of documents -- including one analysis of 52 studies -- from various agency scientists that showed the agency is still split over what to do.

We've reviewed the new data to the extent that we have access to it, Janet Woodcock, director of FDA's Center for Drug Evaluation and Research, told reporters last week.

The FDA panel of outside experts will discuss the data and hear from researchers, patients and public health advocates at the July 13-14 meeting before offering their advice, which could range from leaving the drug on the market and collecting more data to calling for its withdrawal.

But at least one expert witness for the plaintiffs, a Canadian cardiologist, said what he saw was so critical that he sought to testify before the FDA advisers this week. He did not indicate the nature of the documents.

In a letter to the FDA, McGill University's Allan Sniderman said he has access to data that has not previously been available to any similar group but is barred by a court protective order from discussing the documents he has seen.

These data are, in my opinion, essential to review, he wrote last month.

Sniderman did not return calls seeking comment, but Zonies said the cardiologist later withdrew his request. Glaxo spokeswoman Mary Anne Rhyne said the company gave consent for Sniderman to speak with court approval but that he withdrew his request before the court could act.

Rhyne also said Glaxo has consistently said that experts in the litigation may share their views on the data relating to Avandia with FDA and Senator Charles Grassley, the top Republican on the Senate Finance Committee, as long as the judge gave permission.

The once $3-billion-a-year drug has seen its sales slip but it still brings in about $1 billion in annual sales. Glaxo has taken 2 billion pounds ($3 billion) in litigation provisions, but analysts have said the company's liability risk is unclear should Avandia be pulled from the market.

Glaxo has not confirmed the total number of lawsuits it faces. While it has already settled some state lawsuits, others are pending along with federal litigation.

The federal case combines lawsuits filed in various federal district courts and is set to go to trial October 5. Another state case is also scheduled for October.

The federal case is IN RE: Avandia Marketing, Sales Practices and Products Liability Litigation, U.S. District Court, Eastern District of Pennsylvania, MDL No. 1871, No. 07-md-01871.

(Reporting by Susan Heavey; Editing by Tim Dobbyn.)