GlaxoSmithKline PLC could be facing liabilities up to $6 billion over personal injury lawsuits relating to its diabetes drug Avandia, UBS analysts said in a note made public on Friday.

The UBS note, which comes a week after a highly critical report on the drug by the U.S. Senate Finance Committee, said the risk came from more than 13,000 personal injury lawsuits outstanding in the United States.

"As U.S. Avandia contributes 1 percent to sales, our concerns are solely on personal injury lawsuits," analysts said in a note headed by analyst Gbola Amusa.

UBS said the poll of experts suggested Glaxo's liability was in the range of $1 billion to $6 billion.

"We expect liability below the midpoint of this range and note GlaxoSmithKline has underperformed by around $2.5bn already," the note said.

The first of two U.S. federal bellwether trials, which will likely provide an indicator of court decisions on the other cases, is due to start on June 1 in Pennsylvania.

The U.S. Senate report criticized GlaxoSmithKline's handling of heart risks with the drug, charging that Glaxo minimized the drug's safety risks and withheld important data from the U.S. Food and Drug Administration.

The FDA added a warning about potential heart attacks to Avandia in 2007, but the scope of that risk is still not fully understood.

Glaxo has said that the Senate criticism "mischaracterizes and distorts" the company's record.

UBS said that it believes data on Avandia support the drug's continued use, but added that "nonscientific political pressure from Congress will likely be as important as science at FDA in the coming months; and, product withdrawal, if it occurs, could put GlaxoSmithKline at the top end of our liability range."

UBS cut its price target on Glaxo from 1,550 pence to 1,475 pence, but kept its buy recommendation.

The stock closed 0.5 percent lower at 1,238.5 pence on Friday.