“I intersected briefly with her at Oxford,” she wrote.

“As a fresher I wandered into Balliol JCR one day in search of its subsidised breakfast granola-and-Nescafé offering and found a shiny glamazon with naughty eyes holding court astride a table, a high-heeled boot resting on my brother Boris’s thigh.”

Mr Johnson’s sister said she then went to a party at the family home of Ms Maxwell and Robert Maxwell, the media magnate, in north Oxford.

“She gave me a pitying glance but I did manage to snag an invite to her party in Headington Hill Hall — even though I wasn’t in the same college as her and Boris,” Ms Johnson said.

“I have a memory of her father, Bob, coming out in a towelling robe and telling us all to go home.”

In apparent barb directed at her brother, she added that “fairweather friends” of the alleged sex criminal “would not want to reveal they went to a Ghislaine Maxwell party”.

“You only know who your real chums are when you’re in the gutter,” she said.

A photo that purportedly showed Mr Johnson and Ms Maxwell together, published online last year, was later proven to be false.

‘Weak, frail’ Ghislaine Maxwell faces trial

Mr Johnson has never spoken about his connection to the heiress. Downing Street declined to comment on the revelation on Wednesday.

The pair both studied at Balliol, Oxford, where Mr Johnson read classics and she read modern history with languages.