Britain's Labour Party leader Jeremy Corbyn leaves his home in London, Britain June 19, 2019. REUTERS/Peter Nicholls

LONDON (Reuters) - British opposition Labour Party leader Jeremy Corbyn, 70, called for an independent investigation into a Times newspaper report which cited unidentified senior officials as saying they were concerned he was too frail to be prime minister.

“This matter has inevitably undermined confidence in the principle of civil service neutrality, which is integral to the healthy functioning of our democracy,” Corbyn wrote to Cabinet Secretary Mark Sedwill, head of the civil service.

“I would urge you to ensure that there is a speedy and thorough independent investigation, rather than one carried out by the Cabinet Office,” Corbyn wrote.

Prime Minister Theresa May’s spokesman said it was the Cabinet Office’s role to investigate.

“The Cabinet Office is investigating ... if we are able to identify an individual responsible we will take disciplinary action,” he told reporters.

“We are taking the matter extremely seriously. The civil service is responsible for looking into any potential breaches of the civil service code and this is no different.”

The Times reported last month that senior civil servants have become increasingly concerned about Corbyn’s health and warned that he might be forced to stand down as Labour leader because he was not up to the job “physically or mentally”.

“The idea that civil servants should be briefing newspapers against an elected politician, against a prospective government is something that should be very concerning to a lot of people,” Corbyn said on July 1 of the story.

Corbyn has said he is “a very fit, a very healthy and very active person.”