The lioness is quitting her den. In a statement last week Sue Lawley purred she is to resign from Radio 4’s Desert Island Discs. She’ll tape a dozen more shows before the summer, but on August 27 that’s it. After the traditional burst of By the Sleepy Lagoon, listeners will no longer hear that seductive mix of Kathleen Turner and Margaret Thatcher announce, “My castaway today is . . .”

Whoever replaces her — the search has not yet begun, but apparently Paxo fancies the gig — will have big stilettos to fill. Lawley, after all, is the woman who asked Gordon Brown if he was gay, got Ted Heath to say the latter years of his political life had “a certain loneliness and sense of waste” about them and reduced professional northerner Alan Bleasdale to snuffles.

Though her Robinson Crusoes arrive at the studio with the idea that they’ll be choosing eight records, a favourite book and a luxury for island life, Lawley’s interview style (which she proudly describes as “gently probing with an edge”) more often sees her rooting around in unhappy childhoods, tortured love lives and even dodgy personal finances — as a spluttering Ken Dodd found when she quizzed him on his tax problems.

Conversely, while half the population criticises her for being needlessly intrusive, the other half finds her tediously fawning. She oozed platitudes over John Major and David Frost, not to mention getting all hot and bothered with George Clooney. Roy Hattersley once wrote she was a frivolous peek-a-boo inquisitor asking questions of the “chase me round the studio” variety. Diana Plomley, widow of Desert Island Discs’ founding presenter Roy, accused her of having an unsavoury preoccupation with her subjects’ sex lives.

But after 18 years in the chair, Lawley, who turns 60 in July, is unapologetic. “People who come on understand the deal,” she proclaims to me in that familiar clipped tone she used to read the news with. “They know they’re coming to spread their lives out before you, and that means warts and all.”

Since she took over the show in 1988 — her first castaway was Michael Heseltine whose (disturbing) first choice of record was The Teddy Bears’ Picnic — some 750 guests have permitted Lawley to examine their warts. Five prime ministers, umpteen Nobel prize winners, literary giants, sporting heroes, military men, royals, captains of industry and a generous sprinkling of theatrical greats. Famously, the only requirement of those asked on the show is that they be a “ person of achievement”.

Lawley’s own achievement (aside from an OBE in 2001) is to have taken the show to its six-year ratings high. Nearly 2m listeners tune in weekly to hear her unpick the great and good, and their response to her abdication on BBC message boards was incredulous.

“There’s this implication that I’ve somehow deserted my post, isn’t there?” says Lawley, a little bemused. “That I’ve turned my back on that which should not have its back turned on. I suppose I could have gone on a bit longer really,” she ponders. So why is she leaving?

One answer could be boredom. Regular listeners will appreciate the programme can leave the impression that mankind is deeply unimaginative, forever choosing between Beethoven’s Ninth or Debussy’s Clair de Lune. Encyclopedia Britannica and Tolstoy are the most popular book choices (there’s already a complete works of Shakespeare on the island) and rare is the castaway who shocks, as Jamie Oliver did, by telling her he didn’t want a book because he doesn’t like reading.

Lawley admits that many of her interviewees aren’t memorable; interestingly the ones whose stories stick tend not to be the über celebs. “Being a big name doesn’t make you a good interviewee. I mean, I interviewed George Clooney,” she laughs throatily. Was he fun? “Fun while it lasted,” she says. “He’s not got that great a story, though. On the other hand, I’ve interviewed so many Holocaust survivors. Those are the ones that stay with you.”

So why is she giving it up? “Well, I’ve been giving it some serious thought for a year or more now,” she says carefully. “The programme dominates my professional life and it dominates my personal life too. I do 42 programmes a year so I’m always researching, reading a guest’s books or seeing their films and so on.

“It’s not a nine-to-five job. The current controller of Radio 4, my old friend Mark Damazer, wanted me to do even more programmes so I did feel rather like it was taking over and I wanted it to stop.”

Stop? The show only had one real presenter before Lawley and his 43-year reign ended with his death. Though Michael Parkinson took over for 18 months in the mid-1980s he was generally considered not to have been up to snuff, meaning for the best part of two decades Desert Island Discs has defined Lawley as she has defined it. Yet when I ask what she’ll miss about her island paradise, she answers with a deafening 10-second pause . . .