The gesture, known as the doigt d'honneur in French, was made by Socialist MP Henri Emmanuelli during a heated debate in the National Assembly.
Fillon was responding to criticisms from the Socialist Party on the government's fiscal reforms. He said "there is a country that is doing better than us and it's Germany. Why is Germany doing better than us? Because under the government of Mr Schröder, they started early to make the changes that we are currently making".
This seemed to provoke the visibly agitated MP for Landes who then raised his finger in the air. The incident was caught on TV cameras and has been broadcast on TV and around the internet since.
Having realised that the act had been caught on camera, Emmanuelli later told MPs that he had not made the gesture but that he was sorry if that was how it looked. He went on to say that he was "big enough and experienced enough" to be able to say what he wanted to say to the Prime Minister without the help of the doigt d'honneur.
Other MPs were quick to react. Eric Raoult, an MP with the governing UMP party, told Le Parisien, "it's shocking. When you've held high office in the parliament, you shouldn't behave like that. It's the type of thing a schoolboy does in the playground." Others demanded that Emmanuelli should be sanctioned. Christian Jacob, the president of the UMP in parliament, told AFP that he challenged his counterpart, Jean-Marc Ayrault, president of the Socialist group, to "condemn the obscene gesture".