A battery fault has been ruled out as the source of problems on two Japanese Boeing 787 Dreamliners.

Japanese airline safety investigators have shifted their attention to the electrical system that monitors battery voltage, charging and temperature on the new generation aircraft.

But this has raised fears that there will be no quick fix to a problem that meant all 50 787s in service were grounded.

Transport ministry official Shigeru Takano said “we have found no major quality or technical problem” with the lithium-ion batteries.

“We are looking into affiliated parts makers,” the BBC reporting him as saying. “We are looking into possibilities.”

Societe Generale aviation analyst Zafar Khan was reported as saying: “The obvious implication is that it may prolong the grounding.

“If it’s not the battery then we are back to the drawing board. We know it’s an electrical – and not a structural – issue and that will be the focus for the inspectors. But there’s a lot of cabling on these aircraft.”

He said that most analysts had forecast that the 787 would be out of service for eight weeks at most. Beyond 10-12 weeks, and it could impact on Boeing’s production line and future deliveries, he said.

The safety investigation started after a 787 operated by All Nippon Airways made an emergency landing in Japan when its main battery overheated. Earlier, a battery in a Jal Dreamliner caught fire at Boston airport.

US Federal Aviation Administration said two weeks ago that both batteries had leaked electrolyte fluid, and there had been smoke damage to parts of the aircraft.

The FAA said airlines must demonstrate battery safety before flights could resume, a statement that effectively meant airlines had to ground their 787s.

Boeing, which has orders for more than 800 Dreamliners, has halted 787 deliveries.

Thomson Airways and British Airways are the first UK carriers expecting to take deliver of the aircraft n the next few months.

Boeing said last week that is continued to assist the US National Transportation Safety Board and regulators in the US and Japan responsible for investigating the 787 incidents in the US and Japan.

“The company has formed teams consisting of hundreds of engineering and technical experts who are working around the clock with the sole focus of resolving the issue and returning the 787 fleet to flight status,“ the manufacturer said.

“We are working this issue tirelessly in co-operation with our customers and the appropriate regulatory and investigative authorities.

“In order to ensure the integrity of the process and in adherence to international protocols that govern safety investigations, we are not permitted to comment directly on the ongoing investigations.

“Boeing is eager to see both investigative groups continue their work and determine the cause of these events, and we support their thorough resolution.”