It would have been the most expensive bottle of soft drink in history. A secretary at Coca-Cola’s global headquarters has been arrested for stealing a phial of a secret new product, hiding it in a brown Armani bag and attempting to sell it for $1.5m (£800,000) to the world’s second biggest cola maker, Pepsi.

In an audacious conspiracy which quickly descended into amateurism, Joya Williams, 41, an assistant to Coke’s global brand director, is accused of trying to re-ignite the “cola wars” between the two firms, which have historically clashed rancorously over taste tests, sponsorship, secret ingredients and nutrition.

But Williams catastrophically misjudged the rivals’ present-day relationship. Pepsi reported her to her bosses, who called in the police. Undercover FBI officers posed as Pepsi executives and pretended to broker a deal which culminated in a red-handed arrest.

Public prosecutor David Nahmias praised Pepsi for doing the right thing: “They did so because trade secrets are important to everybody in the business community. They realise that if their trade secrets are violated, they all suffer, the market suffers and the community suffers.”

Williams was due to appear in an Atlanta court yesterday charged with unlawfully stealing and selling trade secrets along with two accomplices - Ibrahim Dimson, 30, from New York, and Edmund Duhaney, 41, from Georgia.

The trio are accused of writing a letter to Pepsi, under the pseudonym “Dirk”, in which they demanded an initial payment of $10,000 for Coke internal documents and for the sample.

The conspirators wrote: “I have information that’s all classified and extremely confidential, that only a handful of the top execs at my company have seen. I can even provide actual products and packaging of certain products, that no eye has seen, outside of maybe 5 top execs.”

Once Pepsi had reported the approach, the FBI strung the trio along. According to documents filed by the department of justice, Coke confirmed that goods on offer were valid, “highly confidential” trade secrets and Ms Williams was covertly filmed at work secreting them into her bag.

On June 16, an undercover agent agreed to meet Dimson in Atlanta. Dimson handed over an envelope of documents and the glass phial. In return, he was given a yellow Girl Scout cookie box containing $30,000 with the rest of the money to follow once “testing” had occurred.

The trio opened a bank account to receive the money, registered at Duhaney’s home in the Atlanta suburb of Decatur. But they were arrested on Wednesday, as lawmakers swooped just as the transfer was due to happen.

Coke and Pepsi’s rivalry has long been regarded as among the most bitter in the business world. They compete for big-budget celebrity endorsements - at one point, Coke fielded Michael Jackson against Pepsi’s Madonna. In the 70s, the two ran aggressive campaigns showing “blind tasting” sessions in which their own colas won. There was even an effort to blind taste in space, with both products loaded onto the space shuttle in 1985.

Aggressive marketing has given Coke the edge in global dominance. But Coke’s performance has reached a plateau over the last decade, while Pepsi’s rapid growth has doubled its share price.

Mark Prendergast, author of a book chronicling the history of Coca-Cola, said that this week’s incident was a sign of the firm’s declining mystique: “Traditionally, people who worked at Coca-Cola stayed there for many, many years and treated it as if it were a mission, a religion. It would be inconceivable for anyone who worked at Coke to reveal anything to PepsiCo.”

He said loyalty had been eroded as the two firms headhunted each others’ executives - although Coca-Cola’s present Irish-born chief executive, Neville Isdell, remains a hardcore company man. Mr Isdell made his name running Coke’s Philippines operation in the 1980s, where he specialised in hosting employee rallies wearing military fatigues. The rallies culminated in him smashing a bottle of Pepsi against a wall.