Hostilities were first engaged on the gleaming facades of Montreuil office park, east of Paris, home to gaming giant Ubisoft and BNP-Paribas bank's IT systems. The conflict has since spread to the business district of La Défense, and to the equally besuited enclave of Issy-les-Moulineaux, which houses France's greatest concentration of telecoms and media companies.

In fact, across the French capital, the summer has been enlivened by a corporate collage contest known as La guerre des Post-it (the Post-it wars). The battle is between presumably underemployed office workers who are devoting large chunks of their days to sticking thousands of pink, yellow, orange and green notes on their windows to recreate pixelated images.

The gauntlet thrown down in Montreuil in May was picked up by GDF/Suez and Société Générale at La Défense, then by other companies such as news channel France 24 and Coca-Cola France in Issy. More recently, Post-it collages have been spotted in the windows of advertising agencies in elegant Paris apartment buildings, and also in Lyons and Lille.

Young executives from competing firms meet at lunchtime to compare their creations and plan ripostes. "Each time we have to come up with something bigger, wackier, more adventurous," Julien Berissi, 28, a project manager at Société Générale, told TF1 news. His rival, Stephane Heude from GDF/Suez, said the battle had been "very good socially", bringing together workers who would not otherwise have met.

For the time being, Ubisoft – where office manager Emilie Cozette is credited with the opening salvo – is the champion: its latest effort extended over three floors and used more than 3,000 Post-it notes to create a a design so complex it had to be worked out on a computer.