(Adds Gilead data from Japanese study)

LONDON, June 16 (Reuters) - Britain's healthcare cost-effectiveness watchdog said on Monday it needed more information about Gilead Sciences' pricey new hepatitis C drug Sovaldi before deciding if it should be used on the state health service.

The National Institute for Health and Care Excellence (NICE) said it was "minded not to recommend" the drug, which is also known as sofosbuvir. The decision poses a hurdle to its widespread adoption in Britain.

"The available evidence shows that sofosbuvir is an effective treatment for chronic hepatitis C in certain patients," said Carole Longson, director of the NICE Centre for Health Technology Evaluation.

"However, evidence is lacking for some subgroups of patients with chronic hepatitis C, and there are also substantial uncertainties in the evidence base presented by the manufacturer."

Meanwhile, Gilead released more data on Sovaldi in combination with its second hepatitis C drug ledipasvir that should make it more difficult for NICE to reject.

In a Japanese study of patients with the most common and often difficult to treat Genotype 1 form of the virus, the Sovaldi/ledipasvir combination demonstrated a 100 percent cure rate - 83 of 83 previously untreated patients and all 88 of those who had not been helped by prior treatment - with 12 weeks of therapy.

Once ledipasvir gains approval, the combination with Sovaldi is expected to become a one pill, once a day treatment regimen sold by Gilead.

Sovaldi is far more effective and better-tolerated than older treatments, but its high cost has provoked criticism from healthcare campaigners and insurers.

The U.S. price for a 12-week course of treatment with Sovaldi is $84,000, or $1,000 for each once-daily pill. The price in Britain has been set lower at around 35,000 pounds ($58,800).

Gilead and the pharmaceutical industry trade group has argued that the price is justified by the near guarantee of a cure, far fewer side effects and the treatment's ability to help patients avoid far more expensive hospitalizations for liver failure and the need for transplants as the disease progresses.

($1 = 0.5956 British Pounds) (Reporting by Ben Hirschler, additional reporting by Bill Berkrot in New York; Editing by Louise Heavens and Grant McCool)