Downing Street has insisted the Cabinet Office will investigate if senior civil servants said Jeremy Corbyn was “too frail” to be prime minister despite demands from the Labour leader for an independent probe.
Number 10 said the civil service was best placed to look into the matter, and that any individual found to have made the remarks would be disciplined.
The controversy erupted after The Times reported at the weekend that it had been told by two senior civil servants that Mr Corbyn, 70, may have to stand down due to health issues.
The Labour leader, in a letter to Cabinet Secretary Sir Mark Sedwill, said the matter had “undermined confidence in the principle of civil service neutrality”, as he called for a probe as he called for an independent investigation.
Prime Minister Theresa May’s official spokesman told a Westminster briefing: “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, the PM’s spokesman said: “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, Mrs May’s spokesman said: “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.”
Mr Trickett said: “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.”
The Times report drew a furious response from Labour, which denounced the comments as a “scurrilous” attempt to undermine the party’s efforts to gain power.
Sir Mark wrote to Mr Corbyn on Monday night, offering to meet the Labour leader, the party said.
In his letter the Cabinet Secretary was said to have expressed his concern at the article and promised to investigate the matter.
In his reply, Mr Corbyn thanked Sir Mark for his “assurances” that he was “concerned by the articles”.
“This matter has inevitably undermined confidence in the principle of civil service neutrality, which is integral to the healthy functioning of our democracy.
“This was made clear by Jon Trickett today in Parliament and endorsed by the Speaker of the House of Commons.
“For there to be trust in any investigation, there need to be assurances on its scope and independence.
“In the light of this, I would urge you to ensure that there is a speedy and thorough independent investigation, rather than one carried out by the Cabinet Office.”