David Cameron made a foray back into politics to criticise his successors in Downing Street over their handling of national security issues and international affairs.

Appearing before the National Security Strategy Committee on Monday, the former prime minister said Theresa May made a “very bad mistake” allowing the role of Cabinet secretary and national security adviser to be merged, with Sir Mark Sedwill holding both roles during her tenure in Downing Street.

“They are two jobs,” Mr Cameron told the committee. “For one person, even if you were a cross of Einstein, Wittgenstein and Mother Teresa, you couldn’t possibly do both jobs and I think that temporarily weakened the National Security Council.”

On Boris Johnson’s decision to scrap the Department for International Development (DfID), which was merged with the Foreign Office last year, Mr Cameron said: “I think abolishing DfID is a mistake too for all sorts of reasons but one of which is actually having the Foreign Office voice around the table and the DfID voice around the table I think is important. They are not necessarily the same thing.

“Can you really expect the foreign secretary to do all of the diplomatic stuff and be able to speak to the development brief as well? That’s quite a task, so I think it is good to have both.”

Mr Cameron also insisted his calling of the Brexit referendum, the result of which forced his resignation, was “properly thought-through” and part of a “ grand strategy” for the UK, not “an afterthought” tagged on to the Conservative election manifesto to placate backbenchers.

Asked about the calling of the referendum on leaving the European Union, the former Tory leader said: “The Brexit referendum was discussed and called in 2013, two years before the general election and three years before the referendum itself”.

But he admitted Mr Johnson had suffered from having to face both Brexit and the “immense challenge of the pandemic”, a dual-pronged assault has afforded the current government less time to consider foreign policy.

“All the former prime ministers – we speak to each other from time to time – we’d all say we had difficult decisions to make and difficult circumstances to face, but nothing like this. This has been the greatest difficulty a government has had to face for 40 or 50 years,” Mr Cameron said.

“So to be fair to the government, they have had these twin challenges to deal with.”

Mr Cameron, who was prime minister from 2010 to 2016, said the Ebola outbreak in west Africa in 2013 helped remind his government about the threat of pandemics but admitted a “mistake” was made when putting in safeguards.

He said: “The mistake that was made was that, in thinking about future pandemics, the focus was very much on influenza rather than on respiratory diseases.

“And I’m sure there will be a big inquiry into what we learnt and all the rest, but I think there was a pretty good flu pandemic plan, but it was a flu plan rather than a respiratory diseases plan.

“More should have been learnt from the experience with Sars and respiratory disease in terms of our own preparedness.”

Mr Cameron also sought to highlight how his stewardship of the National Security Council had brought the Nato bombing mission in Libya to a successful conclusion. But he was not questioned about conflict which has raged in the country, with thousands killed, and Islamist extremism rising,  since he and French president Nicolas Sarkozy instigated the western military intervention leading to the overthrow of Muammar Gaddafi.  

The ex-PM assured the committee he did not want to make a return to politics, referencing Donald Trump rumoured bid for the Republican nomination for the 2024 US presidential election. “Thinking about Donald Trump making a comeback is enough to keep us all spinning over”, he said.