Michel Sapin said it was clear that Britain had been caught by surprise by the vote to leave the European Union.
"It has been a few months since the referendum took place. You might have thought that certain people had prepared for this," he told reporters.
"No-one was prepared. You can see very well that they are improvising, with flip-flopping between accommodating positions... and harder positions, even a 'hard' Brexit."
Sapin said he believed that the apparent lack of a plan to see through Brexit showed that some members of the government "did not want (Brexit) and that it has created debates within the government which are clearly very difficult".
In the keenly awaited speech, May is expected to say she favours a clean break from the EU and not a "half-in, half-out" deal with Brussels.
Tuesday's speech is likely to give further signals that Britain is heading towards a so-called "hard" Brexit -- a full break from the EU which entails leaving the single market in order to have full control over immigration.