(Reuters) - Shares of GSK rose as much as 6.5% on Thursday after the British drugmaker agreed to pay up to $2.2 billion to settle lawsuits in the United States that claimed its discontinued heartburn drug Zantac caused cancer.

The agreement, announced on Wednesday, was far lower than some analyst estimates, including JP Morgan's projection of $3.5 billion. The settlement resolves 80,000 or 93% of the pending cases against the company in the U.S.

Analysts at Jefferies said the settlement of the cases should remove "the majority of Zantac overhang on the stock".

Concerns about lawsuits and potential compensation wiped almost $40 billion off the combined market value of GSK, Sanofi, Pfizer and Haleon, which all also sold the drug, over roughly a week in August 2022.

GSK shares are the top gainer on the FTSE benchmark index on Thursday and on track for their best one-day percentage gain since December 2022.

The company will also pay $70 million to settle a related whistleblower lawsuit filed by a Connecticut laboratory.

GSK admitted no wrongdoing or liability in the settlements, and said the settlements were in the best long-term interest of the company to avoid the risk of continuing litigation.

The Brentford-based company expects to record a 1.8 billion pound ($2.36 billion) charge in its third quarter results due to the settlements, with no impact seen on its investment plans.

It will announce third quarter results on Oct.30.

First approved by U.S. regulators in 1983, Zantac became the world's best-selling medicine in 1988 and one of the first to top $1 billion in annual sales. The drug was sold at different times by pharmaceutical companies GSK, Pfizer, Sanofi and Boehringer Ingelheim.

Ranitidine, which was sold under the Zantac brand name, was pulled off the market in 2020 on concerns it could degrade into NDMA, a carcinogen, over time or when exposed to heat. The recall triggered a spate of lawsuits.

($1 = 0.7642 pounds)

(Reporting by Yamini Kalia in Bengaluru; Editing by Mrigank Dhaniwala and Sharon Singleton)