The UK’s top civil servant has denied a “chatty rat” inquiry into government leaking has been de-prioritised – despite the probe lasting more than five months – but admitted the culprit may never be found.

Cabinet Secretary Simon Case on Monday frustrated MPs’ questions about the progress of the investigation, which has been ongoing since the end of October last year when details of England’s second COVID lockdown emerged in the media before a formal announcement.

Mr Case also failed to give answers on a number of other matters – including the funding of the prime minister’s refurbishment of his Downing Street flat.

Amid MPs’ exasperation with the top Whitehall official, former Labour shadow chancellor John McDonnell accused Mr Case of delivering a “badly-scripted version” of 1980s TV sitcom “Yes Minister”.

Mr Case’s appearance before the House of Commons’ public administration and constitutional affairs committee on Monday had been eagerly-anticipated following explosive allegations made by Boris Johnson’s former chief adviser Dominic Cummings.

In a 1,000-word blogpost, Mr Cummings alleged the prime minister suggested halting the “chatty rat” inquiry as it might have implicated a friend of his fiancee, Carrie Symonds.

Mr Case on Monday added to Downing Street’s previous denial that Mr Johnson had sought to interfere in the inquiry, telling MPs: “In relation to this particular leak, and others – the prime minister has always been clear, very determined to see these inquiries complete.”

And he confirmed that the inquiry is ongoing in a “clear indication that the source or sources haven’t been identified”.

However, suggesting the source of the leak might never be found, Mr Case told the committee: “In the time that has now passed, I think it is probable that the team will not successfully identify the source or sources but work is ongoing.”

He assured MPs the inquiry “at no point” over the last five months had been “in any way deprioritised”.

“It is in the hands of professional investigators who have a range of tools and techniques at their disposal,” he added.

“You’ll understand if I don’t go into those in detail… because that would prejudice future inquires. But I can assure this hasn’t been deprioritised in any way.”

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