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The Cabinet Office will investigate alleged newspaper briefings by civil servants questioning Jeremy Corbyn's health and fitness to be PM.

But Labour said the planned probe was inadequate, and only an external inquiry would "restore confidence".

The party said putting civil servants in charge of the investigation showed "every sign of the establishment closing ranks".

Downing Street insisted the matter was being taken "extremely seriously".

Theresa May's spokesman added that, if identified, those responsible would face "disciplinary action".

Over the weekend, the two civil servants had said Mr Corybn was "too frail" to lead Labour or become PM, and may have to stand down over supposed health issues.

Responding to news that the Cabinet Office would lead an inquiry, Labour's Jon Trickett said "nothing less than a thorough investigation" would suffice.

The shadow Cabinet Office minister added: "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".

The Cabinet Office ensures the effective running of government and serves as its corporate headquarters.

Labour MP Laura Pidcock accused senior civil servants of trying to "sow doubt" about Mr Corbyn.

She said there was "a fear of the Labour agenda" among top mandarins who wanted to thwart a Corbyn-led government.

Downing Street has previously said it would be "unacceptable and inappropriate" for any civil servant to make such claims, and one former head of the civil service said it did "more harm" to its own reputation than to Mr Corbyn's.

Speaking to ´óÏó´«Ã½ Radio 5 live, Lord Kerslake - who advises Labour on preparing for government - said it would take time before trust in the service was "restored" following the incident.

He added it was "absolutely essential the civil service maintains its impartiality" and the leak had "set that back".

"I think Jeremy Corbyn is rightly very unhappy about this story."