The casting couch, once the bane of the film world, is now the standard furniture of the legal profession, according to a young City lawyer who has written an explicit novel about life inside a leading law firm.

Briefs, once dry and dusty case documents prepared for silks by solicitors, are now just as likely to be silky undergarments belonging to female trainees obliged to discard them by lascivious partners on salaries beyond the dreams of avarice.

The affairs, bullying and binge-drinking exposed by Alex Gilmore, the nom de plume of the author of Fish Sunday Thinking, are scandalising the profession.

He depicts a world in which young women are ranked by the size of their breasts rather than their legal acumen, in which sex is a commodity designed to advance careers, and in which a large proportion of workaholic young lawyers blow their pay packets on alcohol. The hunt is now on to identify the 27-year-old author and the firm for which he used to work before switching to a practice in the Midlands.

The mystery is likely to rank with that of Belle de Jour, the anonymous London prostitute whose internet memoirs have been sold to Weidenfeld & Nicolson for £100,000.

Mr Gilmore, who has kept his real name secret in order to protect his lucrative job as a legal beagle and pay his mortgage, has said that he wrote his book — the title reflects the dread of returning to work after the weekend — to deter graduates from joining City law firms.

He said: “Senior partners would talk quite brazenly about girls, comparing their cleavage, their legs, their bottoms.”

Pretty girls are promoted over less attractive but more gifted colleagues on the basis that clients prefer to deal with them.

In one graphic scene of his novel, the author recounts the story of a bonding weekend in which a group of corporate lawyers retire to a bar where they take it in turn to extract a slice of lemon from a trainee’s cleavage with their tongues. In another scene, a lawyer wakes to find two legal secretaries in his bed. The revelations have caused consternation among law firms where sex-obsessed, bullying, £1 million-a-year partners are not uncommon.

The Times understands that Gilmore’s target is not a “Magic Circle” (top five) firm nor one in the top 20 best known names, but a middle-ranking firm within the top 50.

The legal profession website rollonfriday.com, which has encouraged discussion of the issues raised in the book, has pledged not to reveal the culprits.

Some of the more outrageous behaviour of senior lawyers has already ended up before employment tribunals. Tougher penalties to stamp out sexual harassment and bullying at the Bar have been called for for more than five years.

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