The best kept secret in Paris has been unveiled. Allegedly.

We speak not of the codes to the French nuclear deterrent. Nor of the true state of the Sarkozy marriage. We speak of the recipe for a steak sauce. This is a magical recipe, a recipe capable of making Parisians do something which Parisians usually refuse to do. To form an orderly queue.

For almost half a century, Parisians and tourists, rain or shine, summer or winter, lunch or dinner, have been queuing for tables at a restaurant which offers only one choice of main dish: steak and chips.

The secret of the success of the Relais de Venise, or "L'Entrecote" as it is sometimes known, is the sauce served with the steak. This is a sauce the colour of pea soup, which is at once sharp and strangely gentle, tangy and sweet.

The recipe for this sauce is a family secret which has remained secret for 48 years.

Until now. In a piece of daring investigative journalism, the Le Monde restaurant critic, Jean-Claude Ribaut, has subjected the sauce to chemical and culinary analysis. He claims to have penetrated the secret of Le Relais de Venise (271, Boulevard Pereire, at Porte Maillot in the 17th arrondissement) at last.

What is the magic ingredient which makes people queue in the rain outside a restaurant which has only one main dish and takes no reservations? The answer he says is "Chicken livers".

The sauce at the Relais de Venise - which now has a franchise in London - is composed, he says, of chicken livers, thyme, thyme flowers, white mustard, single cream, butter, water, salt and pepper. Unlike most French sauces, there are no eggs, no flour, no vegetables. M. Ribault says the secret of the sauce lies partly in how you make it. Dexterity is needed. And practice. (Details below). I called Hélène Godillot, the owner of Le Relais de Venise. It was her father, Paul Gineste de Saurs, who started the restaurant in 1959. He took over a defunct Italian restaurant and quite liked the Gondola scenes painted on the walls. He decided to keep them. Hence the name: Le Relais de Venise (The Venice Inn).

The Gondolas are still there (replicated in the restaurant in London, which opened in 2005). The sauce introduced by Mme Godillot's father is still there (also faithfully replicated in London). Is Mme Godillot not furious that her family secret has been exposed?

"All I will say to you is 'try it'," Mme Godillot told me politely. "Just try the recipe that they give in Le Monde. I defy anyone to make our sauce with that recipe. I don't know where he gets that information. It has no relation to our sauce. Our secret remains intact."

Here, then, for what it is worth, is Le Monde's version of the Relais de Venise steak sauce. No quantities are given. You just have to guess or experiment.

"Whiten the chicken livers with fresh thyme and then let them darken slightly. On a gentle heat, reduce the single cream with white Dijon mustard and flavour with fresh thyme flowers. Put the chicken livers in a mixer and then blend them through a chinois (conical strainer) into the cream and mustard."

"When the sauce thickens, add butter and a little water and salt and pepper."

If you get it right, a queue will form outside your kitchen.

* A mysterious and disturbing incident occurred the other day at the Paris offices of the BBC (also home of The Independent).

A man in a trenchcoat was lurking on the stairs. He seemed uncertain why he was there. When the door was left ajar for a moment, he blocked it with his foot and shouted: "Vive, Gordon Brown" (pronounced in French "broon").

The French Brownite was later identified as Jean-François Probst, 58, former spin-doctor to President Jacques Chirac.