Labour has denounced the UK Government for refusing to launch a full independent inquiry into alleged claims by senior civil servants that Jeremy Corbyn is too frail and too old to be prime minister.

Downing Street made clear the Cabinet Office would investigate the matter, arguing that the civil service was best placed to do so and that any individual found to have made the remarks would be disciplined. No 10, however, declined to say if it was a sackable offence.

But Jon Trickett, the Shadow Cabinet Office Minister, complained, saying: “The refusal of the Government to launch an independent investigation into this clear and unprecedented breach of civil service neutrality at a senior level, shows every sign of the Establishment closing ranks to protect its own.

“When senior civil servants, who are obliged to be impartial, instead spread disinformation about the Leader of the Opposition, it is an attack on our democratic system.

“In these circumstances, nothing less than a thorough investigation, independent of the Cabinet Office, will restore confidence,” he added.

The row broke at the weekend when The Times reported that it had been told by two senior Whitehall officials that 70-year-old Mr Corbyn might have to stand down due to health issues. It was even suggested he had had a minor stroke, a claim strenuously denied by Labour. The party denounced the claims as "scurrilous" while colleagues of the leader insisted he was "as fit as a fiddle".

The officials were also said to have expressed concerns the Labour leader seemed to lack a firm grasp of issues and was being “propped up” by his advisers; claims again denied by Labour HQ.

An indignant Mr Corbyn fired off a letter to Sir Mark Sedwill, the Cabinet Secretary, saying the matter had "undermined confidence in the principle of civil service neutrality" and called for an independent investigation. Sir Mark has offered to meet the Labour leader, the party said.

But Theresa May’s spokesman responded by saying: "The Cabinet Office is investigating this potential breach of the civil service code fully and fairly just as it would any other. If we are able to identify an individual responsible, we will take disciplinary action."

Asked about the Labour leader's call for an independent probe, he replied: "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."

Pressed on the neutrality of a civil service investigation, the spokesman added: "It is alleged the comments were made at a particular event and the Cabinet Office will ensure that it is not carried out by civil servants who are connected to that particular event."