Boris Johnson wrote on a private cabinet paper that David Cameron is a “girly swot”, according to a leaked document obtained by Sky News.
Partially-redacted cabinet papers were released by a court on Thursday showing the prime minister’s reasoning for suspending parliament until mid-October from next week.
Sky News can now publish the unredacted part of the papers, revealing Mr Johnson’s view of his predecessor and suggesting how the prime minister communicates in private.
The handwritten note from Mr Johnson was written on 16 August and was a response to the case made by one of his senior aides to send away MPs for five weeks.
Mr Johnson wrote: “The whole September session is a rigmarole introduced by girly swot Cameron and show the public that MPs are earning their crust.”
Previously, the phrase “by girly swot Cameron” had been redacted in the papers released by the court.
It is thought that they were redacted to minimise political embarrassment. Both politicians went to Eton and were members of the Bullingdon Club for Oxford University students.
Labour’s deputy leader Tom Watson tweeted of the “girly swat” remark: “It’s like this particular cohort of Bullingdon Club and Old Etonians hit their emotional plateau before graduation from Oxford. Johnson and Rees Mogg in particular see politics as a game.”
The latest episode comes a day after Mr Johnson appeared to call Labour leader Jeremy Corbyn a “great big girl’s blouse” in the House of Commons.
The Johnson note emerged during legal action at the Court of Session in Edinburgh, aimed at halting a parliamentary suspension.
In the note marked “sensitive” and “Downing Street only”, the prime minister defended proroguing parliament.
The memo, entitled “ending the session”, was sent to him on 15 August by his senior legal adviser, Nikki da Costa.
In his 16 August reply, he said a suspension would not be “anything especially shocking”, especially as it would be during the political conference season.
The number of “sitting days lost” would be “actually very few”, Mr Johnson said.
The memo is controversial because on 25 August, Downing Street described reports that the prime minister would seek to prorogue parliament as “entirely false”.
But a request by the prime minister for a suspension was approved by the Queen three days later.
The court in Edinburgh ruled on Wednesday that the planned prorogation – due to begin on Tuesday – is lawful. An appeal against that decision is under way.
Since he decided to prorogue parliament, Mr Johnson has suffered four defeats in two days, over Brexit and an early general election.
On Thursday, his brother Jo quit as a minister and as an MP, saying there was an “unresolvable tension” between “family loyalty and the national interest”.
Speaking later in Yorkshire, the PM said he would “rather be dead in a ditch” than delay Brexit.
Opposition party leaders on Friday agreed to once again block Mr Johnson’s demand for a snap general election, meaning the UK is unlikely to head to the polls before November.
It comes ahead of Mr Cameron’s book tour for his forthcoming memoirs.
In 2014, Mr Johnson called Mr Cameron a girly swot in an interview with the Daily Mail, because Mr Cameron got a first-class degree at Oxford University whereas Mr Johnson got a lesser degree mark.
Number 10 did not comment.